Adopt awesome axil idea

doc/api/pipelines.md

@@ -17,8 +17,8 @@ GET /projects/:id/pipelines
 | `yaml_errors`| boolean  | no       | Returns pipelines which have an error of gitlab-ci.yml |
 | `name`| string  | no       | The name of user who triggered pipelines |
 | `username`| string  | no       | The username of user who triggered pipelines |
-| `order_by`| string  | no       | Return requests ordered by `id`, `status`, `ref`, or `user_id` fields. Default is `id` |
-| `sort`    | string  | no       | Return requests sorted in `asc` or `desc` order. Default is `desc` |
+| `order_by`| string  | no       | Return requests ordered by `id`, `status`, `ref`, or `user_id`. Default is `id`. Can be combined with `sort`. If you omit `sort`, its default value is used (`desc`) |
+| `sort`    | string  | no       | Return requests sorted in `asc` or `desc` order. Default is `desc`. Can be combined with `order_by`. If you omit `order_by`, its default value is used (`id`) |
 
 ```
 curl --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" "https://gitlab.example.com/api/v4/projects/1/pipelines"