f add comment as to why we do a double-pass

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -8386,6 +8386,15 @@ where
 			retry_lock: Mutex::new(())
 		};
 
+		// We have to replay (or skip, if they were completed after we wrote the `ChannelManager`)
+		// each `ChannelMonitorUpdate` in `in_flight_monitor_updates`. After doing so, we have to
+		// check that each channel we have isn't newer than the latest `ChannelMonitorUpdate`(s) we
+		// replayed, and for each monitor update we have to replay we have to ensure there's a
+		// `ChannelMonitor` for it.
+		//
+		// In order to do so we first walk the all of our live channels (so that we can check their
+		// state immediately after doing the update replays, when we have the `update_id`s
+		// available) and then walk any remaining in-flight updates.
 		let mut pending_background_events = Vec::new();
 		for (counterparty_id, peer_state_mtx) in per_peer_state.iter_mut() {
 			let mut peer_state_lock = peer_state_mtx.lock().unwrap();