Make tty-menu-open-use-tmm a defcustom, update tmm docs accordingly.

lisp/menu-bar.el

@@ -2340,9 +2340,16 @@ If nil, the current mouse position is used."
   "Keymap used while processing TTY menus.")
 
 
-;; FIXME: Make this a defcustom!
-(defvar tty-menu-open-use-tmm nil
-  "If non-nil, menu-bar-open on a TTY will invoke `tmm-menubar'.")
+(defcustom tty-menu-open-use-tmm nil
+  "If non-nil, \\[menu-bar-open] on a TTY will invoke `tmm-menubar'.
+
+If nil, \\[menu-bar-open] will drop down the menu corresponding to the
+first (leftmost) menu-bar item; you can select other items by typing
+\\[forward-char], \\[backward-char], \\[right-char] and \\[left-char]."
+  :type '(choice (const :tag "F10 drops down TTY menus" nil)
+		 (const :tag "F10 invokes tmm-menubar" t))
+  :group 'display
+  :version "24.4")
 
 (defvar tty-menu--initial-menu-x 10
   "X coordinate of the first menu-bar menu dropped by F10.

lisp/tmm.el

@@ -50,7 +50,11 @@
   "Text-mode emulation of looking and choosing from a menubar.
 See the documentation for `tmm-prompt'.
 X-POSITION, if non-nil, specifies a horizontal position within the menu bar;
-we make that menu bar item (the one at that position) the default choice."
+we make that menu bar item (the one at that position) the default choice.
+
+Note that \\[menu-bar-open] by default drops down TTY menus; if you want it
+to invoke `tmm-menubar' instead, customize the variable
+\`tty-menu-open-use-tmm' to a non-nil value."
   (interactive)
   (run-hooks 'menu-bar-update-hook)
   ;; Obey menu-bar-final-items; put those items last.

src/menu.c

@@ -1064,7 +1064,10 @@ Value is the symbol of the menu at X/Y, or nil if the specified
 coordinates are not within the FRAME's menu bar.  The symbol can
 be used to look up the menu like this:
 
-     (lookup-key global-map [menu-bar SYMBOL])
+     (lookup-key MAP [menu-bar SYMBOL])
+
+where MAP is either the current global map or the current local map,
+since menu-bar items come from both.
 
 This function can return non-nil only on a text-terminal frame
 or on an X frame that doesn't use any GUI toolkit.  Otherwise,