rfkill: Move "state" sysfs file back to stable

There is still quite a bit of code using this interface, so we can't
just remove it. Hopefully it will be possible in the future, but since
its scheduled removal date is past 2 years already, we are better having
the documentation reflecting the current state of things.

Signed-off-by: João Paulo Rechi Vita <jprvita@endlessm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>