Reduce playable columns for sorting

doc/api/pipelines.md

@@ -17,7 +17,7 @@ GET /projects/:id/pipelines
 | `yaml_errors`| string  | no       | If true, returns only yaml error pipelines |
 | `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`, `username`, `started_at`, `finished_at`, `created_at` or `updated_at` fields. Default is `id` |
+| `order_by`| string  | no       | Return requests ordered by `id`, `status`, `ref`, `sha`, or `user_id` fields. Default is `id` |
 | `sort`    | string  | no       | Return requests sorted in `asc` or `desc` order. Default is `desc` |
 
 ```

lib/api/pipelines.rb

@@ -22,7 +22,7 @@ module API
         optional :yaml_errors, type: Boolean, desc: 'If true, returns only yaml error pipelines'
         optional :name,     type: String, desc: 'The name of user who triggered pipelines'
         optional :username, type: String, desc: 'The username of user who triggered pipelines'
-        optional :order_by, type: String, values: %w[id status ref username started_at finished_at created_at updated_at], default: 'id',
+        optional :order_by, type: String, values: %w[id status ref sha user_id], default: 'id',
                             desc: 'The order_by which is combined with a sort'
         optional :sort,     type: String, values: %w[asc desc], default: 'desc',
                             desc: 'The sort method which is combined with an order_by'