clarify limitations of def receive in Scala, see #1842

akka-docs/scala/actors.rst

@@ -51,6 +51,16 @@ be able to handle unknown messages then you need to have a default case as in
 the example above. Otherwise an ``akka.actor.UnhandledMessage(message, sender, recipient)`` will be
 published to the ``ActorSystem``'s ``EventStream``.
 
+The result of the :meth:`receive` method is a partial function object, which is
+stored within the actor as its “initial behavior”, see `Become/Unbecome`_ for
+further information on changing the behavior of an actor after its
+construction. 
+
+.. note::
+   The initial behavior of an Actor is extracted prior to constructor is run,
+   so if you want to base your initial behavior on member state, you should
+   use ``become`` in the constructor.
+
 Creating Actors with default constructor
 ----------------------------------------