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/scala/code/docs/camel/CustomRouteExample.scala

@@ -44,8 +44,8 @@ object CustomRouteExample {
     // example from a MicroKernel
     val system = ActorSystem("some-system")
     val producer = system.actorOf(Props[Producer1])
-    val mediator = system.actorOf(Props(new Transformer(producer)))
-    val consumer = system.actorOf(Props(new Consumer3(mediator)))
+    val mediator = system.actorOf(Props(classOf[Transformer], producer))
+    val consumer = system.actorOf(Props(classOf[Consumer3], mediator))
     CamelExtension(system).context.addRoutes(new CustomRouteBuilder)
     //#CustomRouteExample
   }