runtime(doc): clarify complete_match() and 'isexpand' option

clarify complete_match() documentation to better explain its backward
search behavior, argument handling, and return value format and add an
example of isexpand

closes: #17212

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 03a6a6c..255b9d5 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 Apr 24
+*options.txt*	For Vim version 9.1.  Last change: 2025 Apr 27
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -4996,6 +4996,10 @@
 	Note: Use "\\," to add a literal comma as trigger character, see
 	|option-backslash|.
 
+	Examples: >
+		set isexpand=.,->,/*,\\,
+<
+
 			*'insertmode'* *'im'* *'noinsertmode'* *'noim'*
 'insertmode' 'im'	boolean	(default off)
 			global