Mention that the order-preserving aspect of the new dict

implementation is an implementation detail (and why that is so).

Doc/whatsnew/3.6.rst

@@ -416,7 +416,14 @@ Some smaller changes made to the core Python language are:
 * :func:`dict` now uses a "compact" representation `pioneered by PyPy
   <https://morepypy.blogspot.com/2015/01/faster-more-memory-efficient-and-more.html>`_.
   :pep:`468` (Preserving the order of ``**kwargs`` in a function.) is
-  implemented by this.  (Contributed by INADA Naoki in :issue:`27350`. Idea
+  implemented by this. The order-preserving aspect of this new
+  implementation is considered an implementation detail and should
+  not be relied upon (this may change in the future, but it is desired
+  to have this new dict implementation in the language for a few
+  releases before changing the language spec to mandate
+  order-preserving semantics for all current and future Python
+  implementations).
+  (Contributed by INADA Naoki in :issue:`27350`. Idea
   `originally suggested by Raymond Hettinger
   <https://mail.python.org/pipermail/python-dev/2012-December/123028.html>`_.)