bug#27490: 24.5; Elisp manual explanation of `function-key-map' and its inheritors

[Drew Adams]

Nothing is said about `function-key-map' and its inheritance relations,
beyond the fact that local such maps inherit from `f-k-m', and that
`f-k-m' "should not be used directly" (no reason given).

Please consider explaining `function-key-map'.

In http://lists.gnu.org/archive/html/emacs-devel/2017-06/msg00595.html,
for example, Stefan counsels to "just use function-key-map to translate
[touch-event] to []".  It's not clear how a reader of the manual would
come to such an understanding.

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'