f clarify the height calculation criteria.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -3061,6 +3061,10 @@ impl<ChanSigner: ChannelKeys, M: Deref + Sync + Send, T: Deref + Sync + Send, K:
 
 			channel_state.claimable_htlcs.retain(|&(ref payment_hash, _), htlcs| {
 				htlcs.retain(|htlc| {
+					// If height is approaching the number of blocks we think it takes us to get
+					// our commitment transaction confirmed before the HTLC expires, plus the
+					// number of blocks we generally consider it to take to do a commitment update,
+					// just give up on it and fail he HTLC.
 					if height >= htlc.cltv_expiry - CLTV_CLAIM_BUFFER - LATENCY_GRACE_PERIOD_BLOCKS {
 						timed_out_htlcs.push((HTLCSource::PreviousHopData(htlc.prev_hop.clone()), payment_hash.clone(), htlc.value));
 						false