Complete PyMODINIT_FUNC doc

Author: vstinner

Doc/c-api/intro.rst

@@ -107,9 +107,27 @@ complete listing.
 
 .. c:macro:: PyMODINIT_FUNC
 
-   Declare an extension module ``PyInit`` initialization function. The return
-   type is :c:expr:`PyObject*`. The macro should only be used in the Python C
-   API.
+   Declare an extension module ``PyInit`` initialization function. The function
+   return type is :c:expr:`PyObject*`. The macro declares any special linkage
+   declarations required by the platform, and for C++ declares the function as
+   ``extern "C"``.
+
+   The initialization function must be named :c:func:`!PyInit_name`, where
+   *name* is the name of the module, and should be the only non-\ ``static``
+   item defined in the module file. Example::
+
+       static struct PyModuleDef spam_module = {
+           PyModuleDef_HEAD_INIT,
+           .m_name = "spam",
+           ...
+       };
+
+       PyMODINIT_FUNC
+       PyInit_spam(void)
+       {
+           return PyModule_Create(&spam_module);
+       }
+
 
 .. c:macro:: Py_ABS(x)