patch 9.1.1056: Vim doesn't highlight to be inserted text when completing

Problem:  Vim doesn't highlight to be inserted text when completing
Solution: Add support for the "preinsert" 'completeopt' value
          (glepnir)

Support automatically inserting the currently selected candidate word
that does not belong to the latter part of the leader.

fixes: #3433
closes: #16403

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 880ab0d..648b18c 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 Jan 26
+*options.txt*	For Vim version 9.1.  Last change: 2025 Jan 29
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2168,6 +2168,12 @@
 		    scores when "fuzzy" is enabled. Candidates will appear
 		    in their original order.
 
+	   preinsert
+		    Preinsert the portion of the first candidate word that is
+		    not part of the current completion leader and using the
+		    |hl-ComplMatchIns| highlight group. Does not work when
+		    "fuzzy" is also included.
+
 					*'completepopup'* *'cpp'*
 'completepopup' 'cpp'	string (default empty)
 			global