f explain our guessing on is-from-on-chain

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -8902,6 +8902,9 @@ where
 		}
 
 		for (source, preimage, downstream_value, downstream_closed, downstream_chan_id) in pending_claims_to_replay {
+			// We use `downstream_closed` in place of `from_onchain` here just as a guess - we
+			// don't remember in the `ChannelMonitor` where we got a preimage from, but if the
+			// channel is closed we just assume that it probably came from an on-chain claim.
 			channel_manager.claim_funds_internal(source, preimage, Some(downstream_value),
 				downstream_closed, downstream_chan_id, true);
 		}