Input: xpad - update docs to reflect current state

The last time this documentation was accurate was
just over 8 years ago. In this time we've added
support for two new generations of Xbox console
controllers and dozens of third-party controllers.

This patch unifies terminology and makes it explicit
which model of controller a sentence refers to.

It also expands certain sections to address the latest
versions of Xbox controllers.

Thus this documentation should now be useful to
end users and not contain out-right untruths.

This is the patch's second revision. Prior versions
of this patch altered the driver's TODO list. That
change has been pulled out of this documentation
update patch.

Signed-off-by: Daniel Dressler <danieru.dressler@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>