Merge branch 'patch-250' into 'master'

Add 429 API status code to docs

Closes #228474

See merge request gitlab-org/gitlab!43790

doc/api/README.md

@@ -310,7 +310,7 @@ The following table gives an overview of how the API functions generally behave.
 The following table shows the possible return codes for API requests.
 
 | Return values            | Description |
-| ------------- | ----------- |
+| ------------------------ | ----------- |
 | `200 OK`                 | The `GET`, `PUT` or `DELETE` request was successful, the resource(s) itself is returned as JSON. |
 | `204 No Content`         | The server has successfully fulfilled the request and that there is no additional content to send in the response payload body. |
 | `201 Created`            | The `POST` request was successful and the resource is returned as JSON. |
@@ -323,7 +323,8 @@ The following table shows the possible return codes for API requests.
 | `409 Conflict`           | A conflicting resource already exists, e.g., creating a project with a name that already exists. |
 | `412`                    | Indicates the request was denied. May happen if the `If-Unmodified-Since` header is provided when trying to delete a resource, which was modified in between. |
 | `422 Unprocessable`      | The entity could not be processed. |
-| `500 Server Error` | While handling the request something went wrong server-side. |
+| `429 Too Many Requests`  | The user exceeded the [application rate limits](../administration/instance_limits.md#rate-limits). |
+| `500 Server Error`       | While handling the request, something went wrong server-side. |
 
 ## Pagination