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/option.h b/src/option.h
index e747add..a3634a6 100644
--- a/src/option.h
+++ b/src/option.h
@@ -775,7 +775,7 @@
 #ifdef FEAT_MENU
 EXTERN long	p_mis;		// 'menuitems'
 #endif
-EXTERN char_u	*p_meo;		// 'messagesopt'
+EXTERN char_u	*p_mopt;	// 'messagesopt'
 #ifdef FEAT_SPELL
 EXTERN char_u	*p_msm;		// 'mkspellmem'
 #endif