message: Adding notes about using a custom callbackUrl with the GitLab Auth provider
diff: @@ -35,6 +35,8 @@ auth:
  clientSecret: ${AUTH_GITLAB_CLIENT_SECRET}
  ## uncomment if using self-hosted GitLab
  # audience: https://gitlab.company.com
  + ## uncomment if using a custom redirect URI
  + # callbackUrl: https://${BASE_URL}/api/auth/gitlab/handler/frame
  ```
  The GitLab provider is a structure with three configuration keys:
  @@ -44,6 +46,9 @@ The GitLab provider is a structure with three configuration keys:
  - `clientSecret`: The Application secret
  - `audience` (optional): The base URL for the self-hosted GitLab instance, e.g.
  `https://gitlab.company.com`
  +- `callbackUrl` (optional): The URL matching the Redirect URI registered when creating your GitLab OAuth App, e.g.
  + `https://$backstage.acme.corp/api/auth/gitlab/handler/frame`
  + Note: Due to a peculiarity with GitLab OAuth, ensure there is no trailing `/` after 'frame' in the URL.
  ## Adding the provider to the Backstage frontend
  