f add a bit more color on why certain things are post-shutdown

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -3237,6 +3237,10 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 					|ChannelMonitorUpdateStep::ShutdownScript { .. }
 					|ChannelMonitorUpdateStep::CommitmentSecret { .. } =>
 						is_pre_close_update = true,
+				// After a channel is closed, we don't communicate with our peer about it, so the
+				// only things we will update is getting a new preimage (from a different channel)
+				// or being told that the channel is closed. All other updates are generated while
+				// talking to our peer.
 				ChannelMonitorUpdateStep::PaymentPreimage { .. } => {},
 				ChannelMonitorUpdateStep::ChannelForceClosed { .. } => {},
 			}