Document the change.

Author: ericsnowcurrently

Doc/c-api/typeobj.rst

@@ -1930,7 +1930,14 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
 .. c:member:: PyObject* PyTypeObject.tp_subclasses
 
-   List of weak references to subclasses.  Internal use only.
+   The collection of weak references to subclasses.  Internal use only.
+
+   .. versionchanged:: 3.12
+
+      Internals detail: For the static builtin types this field no longer
+      holds the subclasses.  Those are now stored on ``PyInterpreterState``.
+      This field is re-purposed to hold the index into the type's storage
+      on each interpreter state.
 
    **Inheritance:**
 

Doc/whatsnew/3.12.rst

@@ -413,6 +413,14 @@ Porting to Python 3.12
   ``Py_UNICODE*`` based format (e.g. ``u``, ``Z``) anymore. Please migrate
   to other formats for Unicode like ``s``, ``z``, ``es``, and ``U``.
 
+* ``tp_subclasses`` is no longer used for any static builtin types.
+  The subclasses are stored internally elsewhere.  However, ``tp_subclasses``
+  may still hold data that will cause a crash if used as an object pointer.
+  This internal-only ``PyTypeObject`` field should not be used.  Use the
+  exist public C-API or Python API to access ``__subclasses__`` instead.
+  We mention this in case anyone someone happens to be accessing the
+  field directly anyway.
+
 Deprecated
 ----------