patch 9.1.0864: message history is fixed to 200

Problem:  message history is fixed to 200
Solution: Add the 'msghistory' option, increase the default
          value to 500 (Shougo Matsushita)

closes: #16048

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 73ff7cc..1aa5ccc 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -4440,7 +4440,8 @@
 			global
 	A history of ":" commands, and a history of previous search patterns
 	is remembered.  This option decides how many entries may be stored in
-	each of these histories (see |cmdline-editing|).
+	each of these histories (see |cmdline-editing| and 'msghistory' for
+	the number of messages to remember).
 	The maximum value is 10000.
 	NOTE: This option is set to the Vi default value when 'compatible' is
 	set and to the Vim default value when 'compatible' is reset.
@@ -5917,6 +5918,12 @@
 	time in msec between two mouse clicks for the second click to be
 	recognized as a multi click.
 
+						*'msghistory'* *'mhi'*
+'msghistory' 'mhi'	number	(default 500)
+			global
+	Determines how many entries are remembered in the |:messages| history.
+	The maximum value is 10000.
+
 						    *'mzquantum'* *'mzq'*
 'mzquantum' 'mzq'	number	(default 100)
 			global