[3.13] document Py_VISIT as a macro in the docs (pythonGH-133688) (python#134187)

document `Py_VISIT` as a macro in the docs (pythonGH-133688)
(cherry picked from commit bb32f3c)

Co-authored-by: da-woods <dw-git@d-woods.co.uk>

Author: miss-islington

Doc/c-api/gcsupport.rst

@@ -180,9 +180,9 @@ provided.  In order to use this macro, the :c:member:`~PyTypeObject.tp_traverse`
 must name its arguments exactly *visit* and *arg*:
 
 
-.. c:function:: void Py_VISIT(PyObject *o)
+.. c:macro:: Py_VISIT(o)
 
-   If *o* is not ``NULL``, call the *visit* callback, with arguments *o*
+   If the :c:expr:`PyObject *` *o* is not ``NULL``, call the *visit* callback, with arguments *o*
    and *arg*.  If *visit* returns a non-zero value, then return it.
    Using this macro, :c:member:`~PyTypeObject.tp_traverse` handlers
    look like::