[media] cec: document the special unconfigured case

Even when the CEC device is unconfigured due to an invalid physical
address it is still allowed to send a message from 0xf (Unregistered)
to 0 (TV). This is a corner case explicitly allowed by the CEC specification.

Document this corner case.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>