Merge pull request #24941 from hepin1989/deadleater

= actor add clarify message to dead letter listener

akka-actor/src/main/scala/akka/event/DeadLetterListener.scala

@@ -34,6 +34,7 @@ class DeadLetterListener extends Actor {
       val doneMsg = if (done) ", no more dead letters will be logged" else ""
       eventStream.publish(Info(rcp.path.toString, rcp.getClass,
         s"Message [${message.getClass.getName}] $origin to $rcp was not delivered. [$count] dead letters encountered$doneMsg. " +
+          s"If this is not an expected behavior, then [$rcp] may have terminated unexpectedly, " +
           "This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' " +
           "and 'akka.log-dead-letters-during-shutdown'."))
       if (done) context.stop(self)