rst doc update: adding a link to Joystick class from bge.logic.joysticks
(so one can find more about the joystick class without having to search joystick)
code untested by the way. Sorry but I can no longer build sphinx docs here. I will try to fix that later.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54922 954f8c5b-7b00-dc11-b283-0030488c597c