Enforce mailbox types on System actors. See #3273

akka-docs/rst/scala/actors.rst

@@ -837,6 +837,12 @@ actor's state which have the same property. The :class:`Stash` trait’s
 implementation of :meth:`preRestart` will call ``unstashAll()``, which is
 usually the desired behavior.
 
+.. note::
+
+  If you want to enforce that your actor can only work with an unbounded stash,
+  then you should use the ``UnboundedStash`` trait instead.
+
+
 .. _killing-actors-scala:
 
 Killing an Actor