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/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 1b5f809..2dce6c6 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -692,12 +692,12 @@
 
   " Check hit-enter prompt
   call term_sendkeys(buf, ":set messagesopt=hit-enter,history:500\n")
-  call term_sendkeys(buf, ":echo 'foo' | echo 'bar' echo 'baz'\n")
+  call term_sendkeys(buf, ":echo 'foo' | echo 'bar' | echo 'baz'\n")
   call WaitForAssert({-> assert_equal('Press ENTER or type command to continue', term_getline(buf, 6))})
 
   " Check no hit-enter prompt when "wait:" is set
   call term_sendkeys(buf, ":set messagesopt=wait:100,history:500\n")
-  call term_sendkeys(buf, ":echo 'foo' | echo 'bar' echo 'baz'\n")
+  call term_sendkeys(buf, ":echo 'foo' | echo 'bar' | echo 'baz'\n")
   call WaitForAssert({-> assert_equal('                           0,0-1         All', term_getline(buf, 6))})
 
   " clean up