Toggle Footnote mode.
With a prefix argument ARG, enable Footnote mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.
Footnode mode is a buffer-local minor mode. If enabled, it
provides footnote support for `message-mode'. To get started,
play around with the following keys:
Uses keymap `footnote-minor-mode-map', which is not currently defined.
(fn &optional ARG)