runtime(doc): update documentation on tabstop settings

Unify the treatment of tabstop, correct errors and deprecate smarttab
usage.

closes: #17444

Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/options.txt

@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Jun 07
+*options.txt*	For Vim version 9.1.  Last change: 2025 Jun 10
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -7803,11 +7803,13 @@ A jump table for the options with a short description can be found at |Q_op|.
 			global
 	When enabled, the <Tab> key will indent by 'shiftwidth' if the cursor
 	is in leading whitespace.  The <BS> key has the opposite effect.
-	This behaves as if 'softtabstop' is set to the value of 'shiftwidth'.
+	In leading whitespace, this has the same effect as setting
+	'softtabstop' to the value of 'shiftwidth'.
 	This option is reset when 'compatible' is set; it is temporarily
 	disabled when 'paste' is enabled, and restored when 'paste' is turned
 	off.
-	Have a look at section |30.5| of the user guide for detailed
+	NOTE: in most cases, using 'softtabstop' is a better option.  Have a
+	look at section |30.5| of the user guide for detailed
 	explanations on how Vim works with tabs and spaces.
 
 			*'smoothscroll'* *'sms'* *'nosmoothscroll'* *'nosms'*

runtime/doc/quickref.txt

@@ -1,4 +1,4 @@
-*quickref.txt*  For Vim version 9.1.  Last change: 2025 Jun 04
+*quickref.txt*  For Vim version 9.1.  Last change: 2025 Jun 10
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -942,7 +942,7 @@ Short explanation of each option:		*option-list*
 'tabclose'	  'tcl'     which tab page to focus when closing a tab
 'tabline'	  'tal'     custom format for the console tab pages line
 'tabpagemax'	  'tpm'     maximum number of tab pages for |-p| and "tab all"
-'tabstop'	  'ts'	    number of spaces that <Tab> in file uses
+'tabstop'	  'ts'	    number of columns between two tab stops
 'tagbsearch'	  'tbs'     use binary searching in tags files
 'tagcase'	  'tc'      how to handle case when searching in tags files
 'tagfunc'	  'tfu'	    function to get list of tag matches

runtime/doc/usr_25.txt

@@ -1,4 +1,4 @@
-*usr_25.txt*	For Vim version 9.1.  Last change: 2025 Feb 01
+*usr_25.txt*	For Vim version 9.1.  Last change: 2025 Jun 10
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
@@ -215,27 +215,28 @@ An alternative is to filter the text through an external program.  Example: >
 *25.3*	Indents and tabs
 
 Indents can be used to make text stand out from the rest.  The example texts
-in this manual, for example, are indented by eight spaces or a tab.  You would
+in this manual, for example, are indented by eight columns.  You would
-normally enter this by typing a tab at the start of each line.  Take this
+normally enter this by typing <Tab> at the start of each line.  Take this
 text:
 	the first line ~
 	the second line ~
 
-This is entered by typing a tab, some text, <Enter>, tab and more text.
+This is entered by typing <Tab>, some text, <Enter>, <Tab> and more text.
    The 'autoindent' option inserts indents automatically: >
 
 	:set autoindent
 
 When a new line is started it gets the same indent as the previous line.  In
-the above example, the tab after the <Enter> is not needed anymore.
+the above example, pressing the <Tab> key after <Enter> is not needed anymore.
 
 
 INCREASING INDENT
 
-To increase the amount of indent in a line, use the ">" operator.  Often this
+To increase the amount of indent in a line, use the ">" operator, in Normal
-is used as ">>", which adds indent to the current line.
+mode.  Often this is used as ">>", which adds indent to the current line.
+In Insert mode, use <C-t>.
    The amount of indent added is specified with the 'shiftwidth' option.  The
-default value is 8.  To make ">>" insert four spaces worth of indent, for
+default value is 8.  To make ">>" insert four columns worth of indent, for
 example, type this: >
 
 	:set shiftwidth=4
@@ -248,46 +249,27 @@ When used on the second line of the example text, this is what you get:
 "4>>" will increase the indent of four lines.
 
 
-TABSTOP
+SOFT TAB STOPS
 
 If you want to make indents a multiple of 4, you set 'shiftwidth' to 4.  But
-when pressing a <Tab> you still get 8 spaces worth of indent.  To change this,
+when pressing a <Tab> you still get 8 columns worth of indent.  To change
-set the 'softtabstop' option: >
+this, set the 'softtabstop' option: >
 
 	:set softtabstop=4
 
-This will make the <Tab> key insert 4 spaces worth of indent.  If there are
+Vim now creates invisible tab stops for your cursor every 4 columns; hitting
-already four spaces, a <Tab> character is used (saving seven characters in the
+<Tab> jumps to the next stop and inserts the exact mix of spaces or tabs
-file).  (If you always want spaces and no tab characters, set the 'expandtab'
+needed.
-option.)
 
 	Note:
 	You could set the 'tabstop' option to 4.  However, if you edit the
 	file another time, with 'tabstop' set to the default value of 8, it
 	will look wrong.  In other programs and when printing the indent will
 	also be wrong.  Therefore it is recommended to keep 'tabstop' at eight
-	all the time.  That's the standard value everywhere.
+	all the time.  That's the standard value everywhere on UNIX-like
+	systems.
 
 
-CHANGING TABS
-
-You edit a file which was written with a tabstop of 3.  In Vim it looks ugly,
-because it uses the normal tabstop value of 8.  You can fix this by setting
-'tabstop' to 3.  But you have to do this every time you edit this file.
-   Vim can change the use of tabstops in your file.  First, set 'tabstop' to
-make the indents look good, then use the ":retab" command: >
-
-	:set tabstop=3
-	:retab 8
-
-The ":retab" command will change 'tabstop' to 8, while changing the text such
-that it looks the same.  It changes spans of white space into tabs and spaces
-for this.  You can now write the file.  Next time you edit it the indents will
-be right without setting an option.
-   Warning: When using ":retab" on a program, it may change white space inside
-a string constant.  Therefore it's a good habit to use "\t" instead of a
-real tab.
-
 ==============================================================================
 *25.4*	Dealing with long lines
 
@@ -576,6 +558,32 @@ The "gR" command uses Virtual Replace mode.  This preserves the layout:
 
 	inp	0.786	0.534	0.693 ~
 
+
+REFORMATTING TABS IN TABLES
+
+You edit a file that contains tabular data and the original author of the file
+decided to align the tabular data using tab characters (instead of spaces).
+Alas, they were using tab stops separated by 4 columns and Vim's default
+is 8 columns; the table looks wrong!  What can be done?
+  To fix the appearance without modifying the file, adjust the setting
+temporarily: >
+
+	:set tabstop=4
+
+This updates the visual layout, but the file itself remains unchanged.
+  Another possibility is to permanently reformat the file.  For this Vim
+provides the |:retab| command.  First, set 'tabstop' to match original layout
+(as above), then run: >
+
+	:retab 8
+
+The ":retab" command will change 'tabstop' to 8, while changing the text such
+that it looks the same.  It changes spans of white space into tabs and spaces
+for this.  You can now write the file.
+   Warning: When using ":retab" on a program, it may change white space inside
+a string constant.  Therefore it's a good habit to use "\t" instead of a
+real tab.
+
 ==============================================================================
 
 Next chapter: |usr_26.txt|  Repeating

runtime/doc/usr_30.txt

@@ -1,4 +1,4 @@
-*usr_30.txt*	For Vim version 9.1.  Last change: 2025 May 30
+*usr_30.txt*	For Vim version 9.1.  Last change: 2025 Jun 10
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
@@ -547,8 +547,8 @@ tab stops.
 
   If you prefer to have different values for 'shiftwidth' and 'softtabstop',
 you can still do so and use <C-t> to indent with 'shiftwidth'.  Or you can
-use the 'smarttab' option introduced in Vim 5.6, allowing for a unified
+use the 'smarttab' option, allowing for a unified <Tab> key that knows what to
-<Tab> key that knows what to do in the different situations.
+do in the different situations.
 
 
 VARIABLE TAB STOPS

runtime/doc/various.txt

@@ -1,4 +1,4 @@
-*various.txt*   For Vim version 9.1.  Last change: 2025 Jun 07
+*various.txt*   For Vim version 9.1.  Last change: 2025 Jun 10
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -514,7 +514,7 @@ T  *+title*		Setting the window 'title' and 'icon'; Always enabled
 N  *+toolbar*		|gui-toolbar|
 T  *+user_commands*	User-defined commands. |user-commands|
 			Always enabled since 8.1.1210.
-H  *+vartabs*		Variable-width tabstops. |'vartabstop'|
+H  *+vartabs*		Variable-width tab stops. |'vartabstop'|
 T  *+vertsplit*		Vertically split windows |:vsplit|; Always enabled
 			since 8.0.1118.
 T  *+vim9script*	|Vim9| script

runtime/doc/vi_diff.txt

@@ -1,4 +1,4 @@
-*vi_diff.txt*   For Vim version 9.1.  Last change: 2025 Jun 02
+*vi_diff.txt*   For Vim version 9.1.  Last change: 2025 Jun 10
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -1102,7 +1102,7 @@ The following options are supported by Vi: ~
 'shiftwidth'	  'sw'	    number of spaces to use for (auto)indent step
 'showmatch'	  'sm'	    briefly jump to matching bracket if insert one
 'showmode'	  'smd'     message on status line to show current mode
-'tabstop'	  'ts'	    number of spaces that <Tab> in file uses
+'tabstop'	  'ts'	    number of columns between two tab stops
 'taglength'	  'tl'	    number of significant characters for a tag
 'tags'		  'tag'     list of file names used by the tag command
 			    {Vi: default is "tags /usr/lib/tags"}