docs: document 'list' behavior when 'listchars' excludes "tab" (#29360)

(cherry picked from commit 7e65f3757b)

runtime/doc/options.txt

@@ -3834,6 +3834,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	between tabs and spaces and for trailing blanks. Further changed by
 	the 'listchars' option.
 
+	When 'listchars' does not contain "tab" field, tabs are shown as "^I"
+	or "<09>", like how unprintable characters are displayed.
+
 	The cursor is displayed at the start of the space a Tab character
 	occupies, not at the end as usual in Normal mode.  To get this cursor
 	position while displaying Tabs with spaces, use: >vim

runtime/lua/vim/_meta/options.lua

@@ -3820,6 +3820,9 @@ vim.go.lw = vim.go.lispwords
 --- between tabs and spaces and for trailing blanks. Further changed by
 --- the 'listchars' option.
 ---
+--- When 'listchars' does not contain "tab" field, tabs are shown as "^I"
+--- or "<09>", like how unprintable characters are displayed.
+---
 --- The cursor is displayed at the start of the space a Tab character
 --- occupies, not at the end as usual in Normal mode.  To get this cursor
 --- position while displaying Tabs with spaces, use:

src/nvim/options.lua

@@ -4868,6 +4868,9 @@ return {
         between tabs and spaces and for trailing blanks. Further changed by
         the 'listchars' option.
 
+        When 'listchars' does not contain "tab" field, tabs are shown as "^I"
+        or "<09>", like how unprintable characters are displayed.
+
         The cursor is displayed at the start of the space a Tab character
         occupies, not at the end as usual in Normal mode.  To get this cursor
         position while displaying Tabs with spaces, use: >vim