Add SAML MR apporval feature flag to docs

doc/user/project/merge_requests/approvals/settings.md

@@ -114,7 +114,11 @@ When this field is changed, it can affect all open merge requests depending on t
 
 ## Require user re-authentication to approve
 
-> SAML authentication for GitLab.com groups [introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/5981) in GitLab 16.6.
+> Requiring re-authentication by using SAML authentication for GitLab.com groups [introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/5981) in GitLab 16.6 [with a flag](../../../../administration/feature_flags.md) named `ff_require_saml_auth_to_approve`. Disabled by default.
+
+FLAG:
+On self-managed GitLab, by default requiring re-authentication by using SAML authentication is not available. To make it available, an administrator can
+[enable the feature flag](../../../../administration/feature_flags.md) named `ff_require_saml_auth_to_approve`. On GitLab.com, this feature is not available.
 
 You can force potential approvers to first authenticate with either: