patch 9.1.0911: Variable name for 'messagesopt' doesn't match short name

Problem:  Variable name for 'messagesopt' doesn't match short name
          (after v9.1.0908)
Solution: Change p_meo to p_mopt.  Add more details to docs.
          (zeertzjq)

closes: #16182

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/message.c b/src/message.c
index 6b994e5..f0d1806 100644
--- a/src/message.c
+++ b/src/message.c
@@ -1093,7 +1093,7 @@
     int		messages_wait_new = 0;
     int		messages_history_new = 0;
 
-    p = p_meo;
+    p = p_mopt;
     while (*p != NUL)
     {
 	if (STRNCMP(p, MESSAGES_OPT_HIT_ENTER,