[3.13] pythongh-144257: document return values of PyModule_SetDocString (pythonGH-144258)

(cherry picked from commit 3e9a5b0)

Co-authored-by: Nathan Goldbaum <nathan.goldbaum@gmail.com>
Co-authored-by: sobolevn <mail@sobolevn.me>

Author: ngoldbaum

Doc/c-api/module.rst

@@ -529,6 +529,9 @@ objects dynamically. Note that both ``PyModule_FromDefAndSpec`` and
    ``PyModuleDef``, using either ``PyModule_Create`` or
    ``PyModule_FromDefAndSpec``.
 
+   Return ``0`` on success.
+   Return ``-1`` with an exception set on error.
+
    .. versionadded:: 3.5
 
 .. c:function:: int PyModule_AddFunctions(PyObject *module, PyMethodDef *functions)