runtime(doc): clarify the effect of 'smarttab'

closes: #17426

Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/insert.txt b/runtime/doc/insert.txt
index 9ec4682..155eb8f 100644
--- a/runtime/doc/insert.txt
+++ b/runtime/doc/insert.txt
@@ -1,4 +1,4 @@
-*insert.txt*    For Vim version 9.1.  Last change: 2025 Jun 02
+*insert.txt*    For Vim version 9.1.  Last change: 2025 Jun 04
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -529,7 +529,7 @@
 mode: hit "v", move to the end of the block, and type "gq".  See also |gq|.
 
 ==============================================================================
-4. 'expandtab', 'smarttab' and 'softtabstop' options	*ins-expandtab*
+4. 'expandtab', 'softtabstop' and 'smarttab' options	*ins-expandtab*
 
 If the 'expandtab' option is on, spaces will be used to fill the amount of
 whitespace of the tab.  If you want to enter a real <Tab>, type CTRL-V first
@@ -540,13 +540,6 @@
 space at a time.  The original character will be put back for only one space
 that you backspace over (the last one).
 
-							*ins-smarttab*
-When the 'smarttab' option is on, a <Tab> inserts 'shiftwidth' positions at
-the beginning of a line and 'tabstop' positions in other places.  This means
-that often spaces instead of a <Tab> character are inserted.  When 'smarttab'
-is off, a <Tab> always inserts 'tabstop' positions, and 'shiftwidth' is only
-used for ">>" and the like.
-
 							*ins-softtabstop*
 When the 'softtabstop' option is non-zero, a <Tab> inserts 'softtabstop'
 positions, and a <BS> used to delete white space, will delete 'softtabstop'
@@ -561,6 +554,13 @@
 cursor.  You will have to delete 'softtabstop' characters first, and then type
 extra spaces to get where you want to be.
 
+							*ins-smarttab*
+When the 'smarttab' option is on, the <Tab> key indents by 'shiftwidth' if the
+cursor is in leading whitespace.  The <BS> key has the opposite effect. This
+behaves as if 'softtabstop' were set to the value of 'shiftwidth'. This option
+allows the user to set 'softtabstop' to a value other than 'shiftwidth' and
+still use the <Tab> key for indentation.
+
 ==============================================================================
 5. Replace mode				*Replace* *Replace-mode* *mode-replace*
 
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 5c6fbfb..4471b7c 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Jun 02
+*options.txt*	For Vim version 9.1.  Last change: 2025 Jun 04
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -7803,19 +7803,14 @@
 				 *'smarttab'* *'sta'* *'nosmarttab'* *'nosta'*
 'smarttab' 'sta'	boolean	(default off)
 			global
-	When on, a <Tab> in front of a line inserts blanks according to
-	'shiftwidth'.  'tabstop' or 'softtabstop' is used in other places.  A
-	<BS> will delete a 'shiftwidth' worth of space at the start of the
-	line.
-	When off, a <Tab> always inserts blanks according to 'tabstop' or
-	'softtabstop'.  'shiftwidth' is only used for shifting text left or
-	right |shift-left-right|.
-	What gets inserted (a <Tab> or spaces) depends on the 'expandtab'
-	option.  Also see |ins-expandtab|.  When 'expandtab' is not set, the
-	number of spaces is minimized by using <Tab>s.
-	This option is reset when 'paste' is set and restored when 'paste' is
-	reset.
-	NOTE: This option is reset when 'compatible' is set.
+	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'.
+	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
+	explanations on how Vim works with tabs and spaces.
 
 			*'smoothscroll'* *'sms'* *'nosmoothscroll'* *'nosms'*
 'smoothscroll' 'sms'	boolean  (default off)
diff --git a/runtime/doc/quickref.txt b/runtime/doc/quickref.txt
index 349c1af..de902ee 100644
--- a/runtime/doc/quickref.txt
+++ b/runtime/doc/quickref.txt
@@ -1,4 +1,4 @@
-*quickref.txt*  For Vim version 9.1.  Last change: 2025 Apr 06
+*quickref.txt*  For Vim version 9.1.  Last change: 2025 Jun 04
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -918,7 +918,7 @@
 'signcolumn'	  'scl'	    when to display the sign column
 'smartcase'	  'scs'     no ignore case when pattern has uppercase
 'smartindent'	  'si'	    smart autoindenting for C programs
-'smarttab'	  'sta'     use 'shiftwidth' when inserting <Tab>
+'smarttab'	  'sta'     <Tab> in leading whitespace indents by 'shiftwidth'
 'smoothscroll'	  'sms'     scroll by screen lines when 'wrap' is set
 'softtabstop'	  'sts'     number of spaces that <Tab> uses while editing
 'spell'			    enable spell checking