Add comment describing when a completion action can be discarded

In an older PR a reviewer had asked why the discarding of a channel
being blocked on another monitor update is okay if the blocked
channel has since closed. At the time, this was not actually okay -
the monitor updates in the channel weren't moved to the
`ChannelManager` on close so the whole pipeline was busted, but
with the changes in 4041f08 the
handling of channel closes with pending monitor updates is now
correct, and so is the existing code block.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -8870,6 +8870,11 @@ where
 								Some((blocked_node_id, blocked_channel_outpoint, blocking_action)), ..
 						} = action {
 							if let Some(blocked_peer_state) = per_peer_state.get(&blocked_node_id) {
+								// If the channel we were blocking has closed, we don't need to
+								// worry about it - the blocked monitor update should never have
+								// been released from the `Channel` object so it can't have
+								// completed, and if the channel closed there's no reason to bother
+								// anymore.
 								blocked_peer_state.lock().unwrap().actions_blocking_raa_monitor_updates
 									.entry(blocked_channel_outpoint.to_channel_id())
 									.or_insert_with(Vec::new).push(blocking_action.clone());