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 <[e-mail]>
Cc: Dmitry Torokhov <[e-mail]>
Cc: [e-mail]
Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Cc: Johann Deneux <[e-mail]>
Cc: Anssi Hannula <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/input/ff.rst

···
 (as it is already the case), but to really enable the rendering of force
 effects.
 This document only describes the force feedback part of the Linux input
-interface. Please read joystick.txt and input.txt before reading further this
-document.
+interface. Please read joydev/joystick.rst and input.rst before reading further
+this document.
 
 Instructions to the user
 ~~~~~~~~~~~~~~~~~~~~~~~~
···
 something goes wrong.
 
 If you have a serial iforce device, you need to start inputattach. See
-joystick.txt for details.
+joydev/joystick.rst for details.
 
 Does it work ?
 --------------