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/optionstr.c b/src/optionstr.c
index 08b2350..9247433 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -3055,11 +3055,11 @@
     int
 expand_set_messagesopt(optexpand_T *args, int *numMatches, char_u ***matches)
 {
-    static char *(p_meo_values[]) = {"hit-enter", "wait:", "history:", NULL};
+    static char *(p_mopt_values[]) = {"hit-enter", "wait:", "history:", NULL};
     return expand_set_opt_string(
 	    args,
-	    p_meo_values,
-	    ARRAY_LENGTH(p_meo_values) - 1,
+	    p_mopt_values,
+	    ARRAY_LENGTH(p_mopt_values) - 1,
 	    numMatches,
 	    matches);
 }