Document DAST_ADVERTISE_SCAN DAST CI/CD variable

doc/user/application_security/dast/index.md

@@ -752,6 +752,7 @@ You can use CI/CD variables to customize DAST.
 | `DAST_AUTH_REPORT`                  | boolean       | Used in combination with exporting the `gl-dast-debug-auth-report.html` artifact to aid in debugging authentication issues. |
 | `DAST_AUTH_VERIFICATION_SELECTOR`   | selector      | Verifies successful authentication by checking for presence of a selector once the login form has been submitted. Example: `css:.user-photo` |
 | `DAST_AUTH_VERIFICATION_LOGIN_FORM` | boolean       | Verifies successful authentication by checking for the lack of a login form once the login form has been submitted. |
+| `DAST_ADVERTISE_SCAN`               | boolean       | Set to `true` to add a `Via` header to every request sent, advertising that the request was sent as part of a GitLab DAST scan. [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/334947) in GitLab 14.1. |
 
 1. Available to an on-demand DAST scan.