patch 8.1.0487: no menus specifically for the terminal window

Problem:    No menus specifically for the terminal window.
Solution:   Add :tlmenu. (Yee Cheng Chin, closes #3439) Add a menu test.
diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index b6e5023..eae83d4 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -835,13 +835,14 @@
 				right mouse button).  Useful for adjusting the
 				menu for what is under the cursor or mouse
 				pointer.
-				The pattern is matched against a single
-				character representing the mode:
+				The pattern is matched against one or two
+				characters representing the mode:
 					n	Normal
 					v	Visual
 					o	Operator-pending
 					i	Insert
 					c	Command line
+					tl	Terminal
 							*OptionSet*
 OptionSet			After setting an option.  The pattern is
 				matched against the long option name.