bpo-26868: Fix example usage of PyModule_AddObject. (#15725)

* Add a note to the PyModule_AddObject docs.

* Correct example usages of PyModule_AddObject.

* Whitespace.

* Clean up wording.

* 📜🤖 Added by blurb_it.

* First code review.

* Add < 0 in the tests with PyModule_AddObject

Doc/includes/custom.c

@@ -35,6 +35,11 @@ PyInit_custom(void)
         return NULL;
 
     Py_INCREF(&CustomType);
-    PyModule_AddObject(m, "Custom", (PyObject *) &CustomType);
+    if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
+        Py_DECREF(&CustomType);
+        PY_DECREF(m);
+        return NULL;
+    }
+
     return m;
 }