Be more explicit that a node is shutting down as it was marked as DOWN (

#5821)

Co-authored-by: Aaron Stannard <aaron@petabridge.com>

src/core/Akka.Cluster/ClusterDaemon.cs

@@ -2243,7 +2243,7 @@ private void ShutdownSelfWhenDown()
                 {
                     // the reason for not shutting down immediately is to give the gossip a chance to spread
                     // the downing information to other downed nodes, so that they can shutdown themselves
-                    _cluster.LogInfo("Shutting down myself");
+                    _cluster.LogInfo("Node has been marked as DOWN. Shutting down myself");
                     // not crucial to send gossip, but may speedup removal since fallback to failure detection is not needed
                     // if other downed know that this node has seen the version
                     SendGossipRandom(MaxGossipsBeforeShuttingDownMyself);