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/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