runtime(doc): Add documentation style

closes: #17627

Co-authored-by: Phạm Bình An <111893501+brianhuster@users.noreply.github.com>
Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>

CONTRIBUTING.md

@@ -88,23 +88,25 @@ If the maintainer does not respond, contact the [vim-dev][0] mailing list.
 
 If you want to contribute new runtime files for Vim or Neovim, please create a
 PR with your changes against this repository here. For new filetypes, do not forget:
-* to add a new [filetype test][12] (keep it similar to the other filetype tests).
-* all configuration switches should be documented
+
+- to add a new [filetype test][12] (keep it similar to the other filetype tests).
+- all configuration switches should be documented
   (check [filetype.txt][13] and/or [syntax.txt][14] for filetype and syntax plugins)
-* add yourself as Maintainer to the top of file (again, keep the header similar to
+- add yourself as Maintainer to the top of file (again, keep the header similar to
   other runtime files)
-* add yourself to the [MAINTAINERS][11] file.
+- add yourself to the [MAINTAINERS][11] file.
 
 # Translations
 
 Translating messages and runtime files is very much appreciated! These things
 can be translated:
-*   Messages in Vim, see [src/po/README.txt][1]
+
+- Messages in Vim, see [src/po/README.txt][1]
   Also used for the desktop icons.
-*   Menus, see [runtime/lang/README.txt][2]
-*   Vim tutor, see [runtime/tutor/README.txt][3]
-*   Manual pages, see [runtime/doc/\*.1][4] for examples
-*   Installer, see [nsis/lang/README.txt][5]
+- Menus, see [runtime/lang/README.txt][2]
+- Vim tutor, see [runtime/tutor/README.txt][3]
+- Manual pages, see [runtime/doc/\*.1][4] for examples
+- Installer, see [nsis/lang/README.txt][5]
 
 The help files can be translated and made available separately.
 See https://www.vim.org/translations.php for examples.
@@ -113,9 +115,11 @@ See https://www.vim.org/translations.php for examples.
 
 Please have a look at the following [discussion][6], which should give you some
 ideas. Please also check the [develop.txt][7] helpfile for the recommended
-style. Often it's also beneficial to check the surrounding code for the style
+coding style. Often it's also beneficial to check the surrounding code for the style
 being used.
 
+For the recommended documentation style, please check [helphelp.txt][16].
+
 # I have a question
 
 If you have some question on the style guide, please contact the [vim-dev][0]
@@ -139,3 +143,4 @@ mailing list. For other questions please use the [Vi Stack Exchange][8] website,
 [13]: https://github.com/vim/vim/blob/master/runtime/doc/filetype.txt
 [14]: https://github.com/vim/vim/blob/master/runtime/doc/syntax.txt
 [15]: https://en.wikipedia.org/wiki/Developer_Certificate_of_Origin
+[16]: https://github.com/vim/vim/blob/master/runtime/doc/helphelp.txt

runtime/doc/helphelp.txt

@@ -1,4 +1,4 @@
-*helphelp.txt*	For Vim version 9.1.  Last change: 2025 Jun 03
+*helphelp.txt*	For Vim version 9.1.  Last change: 2025 Jul 07
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -410,6 +410,24 @@ and 'tabstop' options and the 'filetype' to "help".  Never set a global option
 in such a modeline, that can have undesired consequences.
 
 
+STYLE
+
+If your Vim has 'modeline' enabled, Vim should follow the preferred style
+automatically when editing built-in help files.
+
+Vim help files should be formatted for a 'textwidth' of 78 characters, so they
+look good in a typical 80 x 24 terminal window.
+
+Use two spaces between the final dot of a sentence of the first letter of the
+next sentence.  Like this.
+
+Use tab characters for aligning content, with a 'tabstop' setting of 8.
+This also helps reduce the file size.
+
+Always use |:retab| after you have finished editing.  Don't blindly use
+|:retab!|, always review what will be changed to avoid unwanted changes.
+
+
 TAGS
 
 To define a help tag, place the name between asterisks ("*tag-name*").  The