bug#24237: 24.5; (elisp)`Extended Menu Items', :filter warning

[Drew Adams <drew.adams@oracle.com>]

The doc for :filter says this about the FILTER-FN:

   Emacs can call this function at any time that it does redisplay or
   operates on menu data structures, so you should write it so it can
   safely be called at any time.

Is this true in general, or only when the extended menu item is put on a
menu?

A common idiom is to make use of a `menu-item' construct with a :filter
to create a conditional _keyboard_ key binding.  In such a case, the
`menu-item' construct is not a real menu item - it is not placed on any
menu.

I'm guessing that in such a case this doc paragraph does not apply.  If
this guess is correct then please correct the paragraph, so that it says
something like "If an extended menu item that uses :filter is placed on
a menu then Emacs can call FILTER-FN when...".

Also, is it really the case that FILTER-FN can be called anytime Emacs
does redisplay?  Shouldn't the doc say only that it can be called
anytime Emacs "operates on menu data structures"?  Does it get called by
redisplay other than when redisplay operates on menu data structures?
In the case mentioned above (binding to a keyboard key), would FILTER-FN
ever be called during redisplay?  I'm guessing that it would not.

In GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --host=i686-pc-mingw32'