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.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/input/ff.rst

@@ -16,8 +16,8 @@ goal is not to support these devices as if they were simple input-only devices
 (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
 ~~~~~~~~~~~~~~~~~~~~~~~~
@@ -36,7 +36,7 @@ should keep a hand on your device, in order to avoid it to break down if
 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 ?
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