[3.12] Fix the PyGetSetDef documentation (GH-116056) (GH-116363)

closure is not a function pointer, it is a user data pointer.
(cherry picked from commit df59401)

Co-authored-by: Serhiy Storchaka <[email protected]>

Author: miss-islington

Doc/c-api/structures.rst

@@ -702,20 +702,20 @@ Defining Getters and Setters
 
    .. c:member:: void* closure
 
-      Optional function pointer, providing additional data for getter and setter.
+      Optional user data pointer, providing additional data for getter and setter.
 
 .. c:type:: PyObject *(*getter)(PyObject *, void *)
 
    The ``get`` function takes one :c:expr:`PyObject*` parameter (the
-   instance) and a function pointer (the associated ``closure``):
+   instance) and a user data pointer (the associated ``closure``):
 
    It should return a new reference on success or ``NULL`` with a set exception
    on failure.
 
 .. c:type:: int (*setter)(PyObject *, PyObject *, void *)
 
    ``set`` functions take two :c:expr:`PyObject*` parameters (the instance and
-   the value to be set) and a function pointer (the associated ``closure``):
+   the value to be set) and a user data pointer (the associated ``closure``):
 
    In case the attribute should be deleted the second parameter is ``NULL``.
    Should return ``0`` on success or ``-1`` with a set exception on failure.