Restore the comment removed in r348745.

LAGG_RLOCK() enters an epoch section, so the comment wasn't stale.

Reported by:	jhb
MFC with:	r348745

sys/net/if_lagg.c

@@ -1956,6 +1956,10 @@ lagg_link_active(struct lagg_softc *sc, struct lagg_port *lp)
 	 */
 
 #ifdef INVARIANTS
+	/*
+	 * This is called with either LAGG_RLOCK() held or
+	 * LAGG_XLOCK(sc) held.
+	 */
 	if (!in_epoch(net_epoch_preempt))
 		LAGG_XLOCK_ASSERT(sc);
 #endif