Docs: Add search support for protected branches API

doc/api/protected_branches.md

@@ -24,6 +24,7 @@ GET /projects/:id/protected_branches
 | Attribute | Type | Required | Description |
 | --------- | ---- | -------- | ----------- |
 | `id` | integer/string | yes | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user |
+| `search` | string | no | Name or part of the name of protected branches to be searched for |
 
 ```shell
 curl --header "PRIVATE-TOKEN: <your_access_token>" 'https://gitlab.example.com/api/v4/projects/5/protected_branches'