gh-144257: document return values and failure conditions of PyModule_SetDocString

Author: ngoldbaum

Doc/c-api/module.rst

@@ -1019,7 +1019,10 @@ or code that creates modules dynamically.
    Set the docstring for *module* to *docstring*.
    This function is called automatically when creating a module from
    ``PyModuleDef`` (such as when using :ref:`multi-phase-initialization`,
-   ``PyModule_Create``, or ``PyModule_FromDefAndSpec``).
+   ``PyModule_Create``, or ``PyModule_FromDefAndSpec``). Returns ``0`` on
+   success and ``-1`` on failure. Failure can happen if creating a Python string
+   from *docstring* fails or if setting the ``__doc__`` attribute on *module*
+   fails.
 
    .. versionadded:: 3.5