Update docs on MonitorEvent::HolderForceClosed

In a96e2fe we renamed
`MonitorEvent::CommitmentTxConfirmed` to `HolderForceClosed` to
better document what actually happened. However, we failed to
update the documentation on the type, which we do here.

Pointed out by @yellowred.

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -132,7 +132,8 @@ pub enum MonitorEvent {
 	/// A monitor event containing an HTLCUpdate.
 	HTLCEvent(HTLCUpdate),
 
-	/// A monitor event that the Channel's commitment transaction was confirmed.
+	/// Indicates the Channel's commitment transaction was broadcasted by us and the channel has
+	/// now been closed.
 	HolderForceClosed(OutPoint),
 
 	/// Indicates a [`ChannelMonitor`] update has completed. See