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>
diff --git a/runtime/doc/vi_diff.txt b/runtime/doc/vi_diff.txt
index b35fe77..f694df8 100644
--- a/runtime/doc/vi_diff.txt
+++ b/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 @@
 '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"}