Add more detail to the PyModule_AddStringMacro docs

Author: dan-wilton

Doc/c-api/module.rst

@@ -651,7 +651,13 @@ state:
 
 .. c:macro:: PyModule_AddStringMacro(module, macro)
 
-   Add a string constant to *module*.
+   Add a string constant to *module*. The name and the value are taken from
+   *macro*. For example ``PyModule_AddStrMacro(module, STRINGLIB_TYPE_NAME)``
+   adds the string constant *STRINGLIB_TYPE_NAME* with the value of
+   *STRINGLIB_TYPE_NAME* to *module*. The string *value* must be
+   ``NULL``-terminated and UTF-8 encoded.
+   Return ``-1`` with an exception set on error, ``0`` on success.
+
 
 .. c:function:: int PyModule_AddType(PyObject *module, PyTypeObject *type)