patch 9.1.0883: message history cleanup is missing some tests

Problem:  message history cleanup is missing some tests
Solution: Add tests, refactor common code into did_set_msghistory()
          (Shougo Matsushita)

closes: #16078

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Milly <milly.ca@gmail.com>
Signed-off-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 1aa5ccc..57d4af8 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2024 Nov 14
+*options.txt*	For Vim version 9.1.  Last change: 2024 Nov 24
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -5923,6 +5923,8 @@
 			global
 	Determines how many entries are remembered in the |:messages| history.
 	The maximum value is 10000.
+	Setting it to zero clears the message history.
+
 
 						    *'mzquantum'* *'mzq'*
 'mzquantum' 'mzq'	number	(default 100)