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).

Author: Antoine Riard

lightning/src/ln/channelmonitor.rs

@@ -134,11 +134,18 @@ pub enum ChannelMonitorUpdateErr {
 	TemporaryFailure,
 	/// Used to indicate no further channel monitor updates will be allowed (eg we've moved on to a
 	/// different watchtower and cannot update with all watchtowers that were previously informed
-	/// of this channel). This will force-close the channel in question (which will generate one
-	/// final ChannelMonitorUpdate which must be delivered to at least one ChannelMonitor copy).
+	/// of this channel).
 	///
-	/// Should also be used to indicate a failure to update the local persisted copy of the channel
-	/// monitor.
+	/// At reception of this error, channel manager MUST force-close the channel and return at
+	/// least a final ChannelMonitorUpdate::ChannelForceClosed which must be delivered to at
+	/// least one ChannelMonitor copy. Revocation secret MUST NOT be released and offchain channel
+	/// update must be rejected.
+	///
+	/// This failure may also signal a failure to update the local persisted copy of one of
+	/// the channel monitor instance.
+	///
+	/// In case of a distributed watchtower deployment, failures sources can be divergent and
+	/// thus only a opaque failure is return back.
 	PermanentFailure,
 }