f struct docs

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -1130,6 +1130,12 @@ impl_writeable_tlv_based_enum!(EventCompletionAction,
 	}
 );
 
+/// The source argument which is passed to [`ChannelManager::claim_mpp_part`].
+///
+/// This is identical to [`MPPClaimHTLCSource`] except that [`Self::counterparty_node_id`] is an
+/// `Option`, whereas it is required in [`MPPClaimHTLCSource`]. In the future, we should ideally
+/// drop this and merge the two, however doing so may break upgrades for nodes which have pending
+/// forwarded payments.
 struct HTLCClaimSource {
 	counterparty_node_id: Option<PublicKey>,
 	funding_txo: OutPoint,
@@ -1150,7 +1156,8 @@ impl From<&MPPClaimHTLCSource> for HTLCClaimSource {
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 /// The source of an HTLC which is being claimed as a part of an incoming payment. Each part is
-/// tracked in [`PendingMPPClaim`].
+/// tracked in [`PendingMPPClaim`] as well as in [`ChannelMonitor`]s, so that it can be converted
+/// to an [`HTLCClaimSource`] for claim replays on startup.
 struct MPPClaimHTLCSource {
 	counterparty_node_id: PublicKey,
 	funding_txo: OutPoint,