Improve :type of defcustom 'context-menu-functions' and add documentation.

Improve :type of defcustom 'context-menu-functions' and add documentation.

* doc/emacs/frames.texi (Menu Mouse Clicks): Describe context-menu-mode
and context-menu-functions instead of suggesting global-set-key.
(Menu Bars): Mention context-menu-mode and context-menu-functions.

* etc/NEWS: Add context-menu-mode and context-menu-functions.

* lisp/mouse.el (context-menu-functions): Use :type with repeat/function-item.
(context-menu-minor, context-menu-undo, context-menu-region)
(context-menu-ffap): Improve docstrings displayed for function-item in
defcustom of context-menu-functions.