Document supported runners?search= argument

doc/api/runners.md

@@ -41,6 +41,7 @@ GET /runners?scope=active
 GET /runners?type=project_type
 GET /runners?status=active
 GET /runners?tag_list=tag1,tag2
+GET /runners?search=gitlab
 ```
 
 | Attribute   | Type           | Required | Description         |
@@ -49,6 +50,7 @@ GET /runners?tag_list=tag1,tag2
 | `type`      | string         | no       | The type of runners to show, one of: `instance_type`, `group_type`, `project_type` |
 | `status`    | string         | no       | The status of runners to show, one of: `active`, `paused`, `online`, `offline` |
 | `tag_list`  | string array   | no       | List of the runner's tags |
+| `search`    | string         | no       | The partial token or description text to match |
 
 ```shell
 curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/runners"