Input: docs - freshen up introduction

Stop saying that API is experimental and that only USB is supported,
acknowledge that evdev is the preferred interface, and remove paragraph
encouraging people sending snail mail to Vojtech :) along with his email.
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/input/event-codes.rst

+.. _input-event-codes:
+
 =================
 Input event codes
 =================

Documentation/input/joydev/joystick-api.rst

+.. _joystick-api:
+
 =====================
 Programming Interface
 =====================

Documentation/input/joydev/joystick.rst

 .. include:: <isonum.txt>
 
+.. _joystick-doc:
+
 Introduction
 ============