f explain more of why no-cp-id is "closed channel"

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -8642,7 +8642,10 @@ where
 						if let HTLCSource::PreviousHopData(_) = htlc_source {
 							if let Some(payment_preimage) = preimage_opt {
 								Some((htlc_source, payment_preimage, htlc.amount_msat,
-									counterparty_opt.is_none(), // i.e. the downstream chan is closed
+									// Check if `counterparty_opt.is_none()` to see if the
+									// downstream chan is closed (because we don't have a
+									// channel_id -> peer map entry).
+									counterparty_opt.is_none(),
 									monitor.get_funding_txo().0.to_channel_id()))
 							} else { None }
 						} else {