PEP 509: mention explicitly that PyDictObject is not part of the stable ABI

pep-0509.txt

@@ -158,6 +158,9 @@ initilized to version ``0``. The version is incremented at each change:
   values are compared by identity, not by their content; the version can
   be incremented multiple times
 
+.. note::
+   The ``PyDictObject`` structure is not part of the stable ABI.
+
 Example using an hypothetical ``dict_get_version(dict)`` function::
 
     >>> d = {}