bpo-23722: Fix docs for future __classcell__ changes. (GH-6999) (GH-7000)

(cherry picked from commit 8ae8e6af)
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Doc/reference/datamodel.rst

@@ -1968,7 +1968,7 @@ current call is identified based on the first argument passed to the method.
    be propagated up to the ``type.__new__`` call in order for the class to be
    initialised correctly.
    Failing to do so will result in a :exc:`DeprecationWarning` in Python 3.6,
-   and a :exc:`RuntimeWarning` in the future.
+   and a :exc:`RuntimeError` in Python 3.8.
 
 When using the default metaclass :class:`type`, or any metaclass that ultimately
 calls ``type.__new__``, the following additional customisation steps are

Doc/whatsnew/3.6.rst

@@ -2261,7 +2261,8 @@ Changes in the Python API
   direct references from methods to the implicit ``__class__`` closure
   variable, the implicit ``__classcell__`` namespace entry must now be passed
   up to ``type.__new__`` for initialisation. Failing to do so will result in
-  a :exc:`DeprecationWarning` in 3.6 and a :exc:`RuntimeWarning` in the future.
+  a :exc:`DeprecationWarning` in Python 3.6 and a :exc:`RuntimeError` in
+  Python 3.8.
 
 Changes in the C API
 --------------------