deprecate closure-taking Props factories, see #3081

- base Props on Deploy, Class and Seq[Any] (i.e. constructor args)
- remove deprecated Props usage from akka-docs sample code
- rewrite UntypedActorDocTestBase
- rewrite Java/Scala doc section on actor creation
- add migration guide entry

akka-docs/rst/java/code/docs/camel/CustomRouteTestBase.java

@@ -11,7 +11,7 @@ public class CustomRouteTestBase {
     //#CustomRoute
     ActorSystem system = ActorSystem.create("some-system");
     Camel camel = CamelExtension.get(system);
-    ActorRef responder = system.actorOf(new Props(Responder.class), "TestResponder");
+    ActorRef responder = system.actorOf(Props.create(Responder.class), "TestResponder");
     camel.context().addRoutes(new CustomRouteBuilder(responder));
     //#CustomRoute
     system.stop(responder);