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/optiondefs.h b/src/optiondefs.h
index f42178b..2d01897 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -1696,7 +1696,7 @@
 			    (char_u *)NULL, PV_NONE, NULL, NULL,
 			    {(char_u *)FALSE, (char_u *)0L} SCTX_INIT},
     {"messagesopt","mopt",  P_STRING|P_ALLOCED|P_VI_DEF|P_ONECOMMA|P_COLON|P_NODUP,
-			    (char_u *)&p_meo, PV_NONE, did_set_messagesopt, expand_set_messagesopt,
+			    (char_u *)&p_mopt, PV_NONE, did_set_messagesopt, expand_set_messagesopt,
 			    {(char_u *)"hit-enter,history:500", (char_u *)NULL} SCTX_INIT},
     {"mkspellmem",  "msm",  P_STRING|P_VI_DEF|P_EXPAND|P_SECURE,
 #ifdef FEAT_SPELL