bpo-27505: Add change notes in module attribute docs (GH-5320)

Make it clear that setting __class__ on a module has worked since 3.5,
but support for __getattr__ and __dir__ on module instances requires 3.7+

Patch by Cheryl Sabella.

Doc/reference/datamodel.rst

@@ -1564,6 +1564,17 @@ a module object to a subclass of :class:`types.ModuleType`. For example::
    the module globals (whether by code within the module, or via a reference
    to the module's globals dictionary) is unaffected.
 
+.. versionchanged:: 3.5
+   ``__class__`` module attribute is now writable.
+
+.. versionadded:: 3.7
+   ``__getattr__`` and ``__dir__`` module attributes.
+
+.. seealso::
+
+   :pep:`562` - Module __getattr__ and __dir__
+      Describes the ``__getattr__`` and ``__dir__`` functions on modules.
+
 
 .. _descriptors: