!str #16520 Add ActorRefSource

* and rename the factory for ActorPublisherSource,
  from Source.apply to Source.actorPublisher

* including internal buffer, with OverflowStrategy

* support to complete/fail stream

akka-docs-dev/rst/scala/code/docs/stream/ActorPublisherDocSpec.scala

@@ -76,7 +76,7 @@ class ActorPublisherDocSpec extends AkkaSpec {
       testActor ! s
 
     //#actor-publisher-usage
-    val jobManagerSource = Source[JobManager.Job](JobManager.props)
+    val jobManagerSource = Source.actorPublisher[JobManager.Job](JobManager.props)
     val ref = Flow[JobManager.Job]
       .map(_.payload.toUpperCase)
       .map { elem => println(elem); elem }