input: Documentation: update related file names in ff.rst

input: Documentation: update related file names in ff.rst

Change other related documentation file names from .txt to .rst
and be more explicit about their paths/locations.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Johann Deneux <johann.deneux@gmail.com>
Cc: Anssi Hannula <anssi.hannula@gmail.com>
Link: https://lore.kernel.org/r/20210302223523.20130-4-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>