Note the buffer slots can be set with PyType_Spec with the unlimited API. (GH-22052)

Follow up to f7c4e23.
(cherry picked from commit 700fa1b)

Co-authored-by: Benjamin Peterson <[email protected]>

Author: miss-islington

Doc/c-api/type.rst

@@ -215,7 +215,8 @@ The following functions and structs are used to create
       * ``Py_nb_add`` to set :c:member:`PyNumberMethods.nb_add`
       * ``Py_sq_length`` to set :c:member:`PySequenceMethods.sq_length`
 
-      The following fields cannot be set using :c:type:`PyType_Spec` and :c:type:`PyType_Slot`:
+      The following fields cannot be set at all using :c:type:`PyType_Spec` and
+      :c:type:`PyType_Slot`:
 
       * :c:member:`~PyTypeObject.tp_dict`
       * :c:member:`~PyTypeObject.tp_mro`
@@ -229,13 +230,21 @@ The following functions and structs are used to create
         (see :ref:`PyMemberDef <pymemberdef-offsets>`)
       * :c:member:`~PyTypeObject.tp_vectorcall_offset`
         (see :ref:`PyMemberDef <pymemberdef-offsets>`)
+
+      The following fields cannot be set using :c:type:`PyType_Spec` and
+      :c:type:`PyType_Slot` under the limited API:
+
       * :c:member:`~PyBufferProcs.bf_getbuffer`
       * :c:member:`~PyBufferProcs.bf_releasebuffer`
 
       Setting :c:data:`Py_tp_bases` may be problematic on some platforms.
       To avoid issues, use the *bases* argument of
       :py:func:`PyType_FromSpecWithBases` instead.
 
+     .. versionchanged:: 3.9
+
+        Slots in :c:type:`PyBufferProcs` in may be set in the unlimited API.
+
    .. c:member:: void *PyType_Slot.pfunc
 
       The desired value of the slot. In most cases, this is a pointer