Improve comment for incommingMu.

Change-Id: I57aefee70a3e790388a99d6ea8562f5f51f10f4c
diff --git a/runtime/internal/rpc/stream/vc/xvc.go b/runtime/internal/rpc/stream/vc/xvc.go
index 43eeb31..14e3a93 100644
--- a/runtime/internal/rpc/stream/vc/xvc.go
+++ b/runtime/internal/rpc/stream/vc/xvc.go
@@ -152,6 +152,9 @@
 		return nil, err
 	}
 	vc.incommingMu.Lock()
+	// We unlock this only AFTER authentication is finished to ensure
+	// that no flows are announced until we're fully initialized.
+	// TODO(mattr): Find a better way.
 	defer vc.incommingMu.Unlock()
 	vc.incomming = incomming