Document the rfkill sysfs ABI

This moves sysfs ABI info from Documentation/rfkill.txt to the
ABI subfolder and reformats it.

This also schedules the deprecated sysfs parts to be removed in
2012 (claim file) and 2014 (state file).

Signed-off-by: Florian Mickler <florian@mickler.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>