runtime(doc): clarify {special} argument for shellescape()

closes: #14770

Signed-off-by: Enno <Konfekt@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/builtin.txt

@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2024 May 07
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 May 15
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -8991,11 +8991,12 @@ shellescape({string} [, {special}])			*shellescape()*
 		Otherwise it will enclose {string} in single quotes and
 		replace all "'" with "'\''".
 
-		When the {special} argument is present and it's a non-zero
-		Number or a non-empty String (|non-zero-arg|), then special
-		items such as "!", "%", "#" and "<cword>" will be preceded by
-		a backslash.  This backslash will be removed again by the |:!|
-		command.
+		The {special} argument adds additional escaping of keywords
+		used in Vim commands. When it is not omitted and a non-zero
+		number or a non-empty String (|non-zero-arg|), then special 
+		items such as "!", "%", "#" and "<cword>" (as listed in 
+		|expand()|) will be preceded by a backslash. 
+		This backslash will be removed again by the |:!| command.
 
 		The "!" character will be escaped (again with a |non-zero-arg|
 		{special}) when 'shell' contains "csh" in the tail.  That is