f add note about channel_update using inbound aliases

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -710,6 +710,11 @@ pub(super) struct Channel<Signer: Sign> {
 	// Our counterparty can offer us SCID aliases which they will map to this channel when routing
 	// outbound payments. These can be used in invoice route hints to avoid explicitly revealing
 	// the channel's funding UTXO.
+	//
+	// We also use this when sending our peer a channel_update that isn't to be broadcasted
+	// publicly - allowing them to re-use their map of SCID -> channel for channel_update ->
+	// associated channel mapping.
+	//
 	// We only bother storing the most recent SCID alias at any time, though our counterparty has
 	// to store all of them.
 	latest_inbound_scid_alias: Option<u64>,