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/io/HTTPServer.scala

@@ -237,6 +237,6 @@ case class OKResponse(body: ByteString, keepAlive: Boolean)
 object Main extends App {
   val port = Option(System.getenv("PORT")) map (_.toInt) getOrElse 8080
   val system = ActorSystem()
-  val server = system.actorOf(Props(new HttpServer(port)))
+  val server = system.actorOf(Props(classOf[HttpServer], port))
 }
 //#main