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/actor/InitializationDocSpecJava.java

@@ -78,7 +78,7 @@ public class InitializationDocSpecJava {
     public void testIt() {
 
         new JavaTestKit(system) {{
-            ActorRef testactor = system.actorOf(new Props(MessageInitExample.class), "testactor");
+            ActorRef testactor = system.actorOf(Props.create(MessageInitExample.class), "testactor");
             String probe = "U OK?";
 
             testactor.tell(probe, getRef());