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/transactor/TransactorDocSpec.scala

@@ -199,6 +199,7 @@ class TransactorDocSpec extends AkkaSpec {
 
     val system = ActorSystem("transactors")
 
+    // FIXME, or remove the whole transactor module, srsly
     lazy val underlyingCounter = new Counter
     val counter = system.actorOf(Props(underlyingCounter), name = "counter")
     val coordinated = Coordinated()(Timeout(5 seconds))