Improve PermanentFailure requiremnts documentation

Improve PermanentFailure requiremnts documentation

Sources of the failure may be multiple in case of distributed watchtower
deployment. In either case, the channel manager must return a final
update asking to its channel monitor(s) to broadcast the lastest state
available. Revocation secret must not be released for the faultive
channel.

In the future, we may return wider type of failures to take more
fine-grained processing decision (e.g if local disk failure and
redudant remote channel copy available channel may still be processed
forward).