clu #17565 Invoke OnMemberRemoved callback when

cluster.shutdown

* must also be done when the listener actor stops before the
  MemberRemoved event has been received
* add test for this
* clarify docs with example that shuts down actor system and
  exit jvm

akka-cluster/src/test/scala/akka/cluster/ClusterSpec.scala

@@ -15,6 +15,7 @@ import akka.cluster.InternalClusterAction._
 import java.lang.management.ManagementFactory
 import javax.management.ObjectName
 import akka.actor.ActorRef
+import akka.testkit.TestProbe
 
 object ClusterSpec {
   val config = """
@@ -93,12 +94,17 @@ class ClusterSpec extends AkkaSpec(ClusterSpec.config) with ImplicitSender {
 
     // this should be the last test step, since the cluster is shutdown
     "publish MemberRemoved when shutdown" in {
+      val callbackProbe = TestProbe()
+      cluster.registerOnMemberRemoved(callbackProbe.ref ! "OnMemberRemoved")
+
       cluster.subscribe(testActor, classOf[ClusterEvent.MemberRemoved])
       // first, is in response to the subscription
       expectMsgClass(classOf[ClusterEvent.CurrentClusterState])
 
       cluster.shutdown()
       expectMsgType[ClusterEvent.MemberRemoved].member.address should ===(selfAddress)
+
+      callbackProbe.expectMsg("OnMemberRemoved")
     }
 
   }