routerrpc: add docs to forward interceptor

lnrpc/routerrpc/router.proto

@@ -1063,6 +1063,14 @@ message ForwardHtlcInterceptResponse {
     // 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.
     map<uint64, bytes> out_wire_custom_records = 8;
 }