f update comment

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -4284,10 +4284,14 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 		};
 		debug_assert!(!sources.is_empty());
 
-		// If we are claiming an MPP payment, we have to take special care to ensure that each
-		// channel exists before claiming all of the payments (inside one lock).
-		// Note that channel existance is sufficient as we should always get a monitor update
-		// which will take care of the real HTLC claim enforcement.
+		// If we are claiming an MPP payment, we check that all channels which contain a claimable
+		// HTLC still exist. While this isn't guaranteed to remain true if a channel closes while
+		// we're claiming (or even after we claim, before the commitment update dance completes),
+		// it should be a relatively rare race, and we'd rather not claim HTLCs that require us to
+		// go on-chain (and lose the on-chain fee to do so) than just reject the payment.
+		//
+		// Note that we'll still always get our funds - as long as the generated
+		// `ChannelMonitorUpdate` makes it out to the relevant monitor we can claim on-chain.
 		//
 		// If we find an HTLC which we would need to claim but for which we do not have a
 		// channel, we will fail all parts of the MPP payment. While we could wait and see if