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/testkit/TestKitSampleTest.java

@@ -53,7 +53,7 @@ public class TestKitSampleTest {
      * if you want to receive actor replies or use Within(), etc.
      */
     new JavaTestKit(system) {{
-      final Props props = new Props(SomeActor.class);
+      final Props props = Props.create(SomeActor.class);
       final ActorRef subject = system.actorOf(props);
 
       // can also use JavaTestKit “from the outside”