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

[Randy Dunlap]

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

Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
Cc: linux-input@xxxxxxxxxxxxxxx
---
where is fftest mentioned here found? I only found one place:
  https://github.com/flosse/linuxconsole/blob/master/utils/fftest.c

 Documentation/input/ff.rst |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- linux-next-20210202.orig/Documentation/input/ff.rst
+++ linux-next-20210202/Documentation/input/ff.rst
@@ -16,8 +16,8 @@ goal is not to support these devices as
 (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 or
 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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