Add a comment briefly describing json_object_object_length()

json_object.h

@@ -215,6 +215,9 @@ extern struct json_object* json_object_new_object(void);
  */
 extern struct lh_table* json_object_get_object(struct json_object *obj);
 
+/** Get the size of an object in terms of the number of fields it has.
+ * @param obj the json_object whose length to return
+ */
 extern int json_object_object_length(struct json_object* obj);
 
 /** Add an object field to a json_object of type json_type_object