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/usr_42.txt b/runtime/doc/usr_42.txt
index 4648361..2901913 100644
--- a/runtime/doc/usr_42.txt
+++ b/runtime/doc/usr_42.txt
@@ -150,7 +150,8 @@
 	:menu!		Insert and Command-line mode
 	:imenu		Insert mode
 	:cmenu		Command-line mode
-	:amenu		All modes
+	:tlmenu		Terminal mode
+	:amenu		All modes (except for Terminal mode)
 
 To avoid that the commands of a menu item are being mapped, use the command
 ":noremenu", ":nnoremenu", ":anoremenu", etc.