make FSM.receive overridable, see #2680

akka-docs/rst/scala/fsm.rst

@@ -124,6 +124,14 @@ obvious that an actor is actually created:
    :include: simple-fsm
    :exclude: fsm-body
 
+.. note::
+
+   The FSM trait defines a ``receive`` method which handles internal messages
+   and passes everything else through to the FSM logic (according to the
+   current state). When overriding the ``receive`` method, keep in mind that
+   e.g. state timeout handling depends on actually passing the messages through
+   the FSM logic.
+
 The :class:`FSM` trait takes two type parameters:
 
  #. the supertype of all state names, usually a sealed trait with case objects