Rename new option to flyspell-use-mouse-3-for-menu

Rename new option to flyspell-use-mouse-3-for-menu

* lisp/textmodes/flyspell.el (flyspell-use-mouse-3-for-menu): Rename
from 'flyspell-correct-on-mouse-3'.  The previous name did not make it
clear that it's about opening a menu.  (Bug#11680)
(flyspell--set-use-mouse-3-for-menu): Rename from
'flyspell--set-correct-on-mouse-3'.  Bind menu to 'down-mouse-3'
instead of 'mouse-3' to be more in line with user expectations.
(flyspell-mode): Use the above new names.
* doc/emacs/fixit.texi (Spelling):
* etc/NEWS: Update documentation to use the new name.

Suggested by Stefan Monnier <monnier@iro.umontreal.ca>.