routerrpc: add docs to forward interceptor

lnrpc/routerrpc/router.pb.go

@@ -3253,6 +3253,14 @@ type ForwardHtlcInterceptResponse struct {
 	// Any custom records that should be set on the p2p wire message message of
 	// the resumed HTLC. This field is ignored if the action is not
 	// RESUME_MODIFIED.
+	//
+	// If this map is populated, it will overwrite any of the wire custom
+	// records set by LND. It is the caller's responsibility to copy any desired
+	// records across. If the map is empty, no action will be taken and existing
+	// custom records will be propagated.
+	//
+	// The API does not currently support deleting custom records, unless they
+	// are overwritten by a new set.
 	OutWireCustomRecords map[uint64][]byte `protobuf:"bytes,8,rep,name=out_wire_custom_records,json=outWireCustomRecords,proto3" json:"out_wire_custom_records,omitempty" protobuf_key:"varint,1,opt,name=key,proto3" protobuf_val:"bytes,2,opt,name=value,proto3"`
 }