gh-107298: Document PyMODINIT_FUNC macro (#109236)

Document PyMODINIT_FUNC macro.

Remove links to PyAPI_FUNC() and PyAPI_DATA() macros since they are
not documented. These macros should only be used to define the Python
C API. They should not be used outside Python code base.

Doc/c-api/intro.rst

@@ -105,6 +105,30 @@ defined closer to where they are useful (e.g. :c:macro:`Py_RETURN_NONE`).
 Others of a more general utility are defined here.  This is not necessarily a
 complete listing.
 
+.. c:macro:: PyMODINIT_FUNC
+
+   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 :samp:`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)
 
    Return the absolute value of ``x``.