Add Audit Event retention policy to documentation

Previously the Audit Event retention policy was not documented. This
adds a section to the Audit Event documentation to explain the retention
policy, which only applies to SaaS customers.

doc/administration/audit_events.md

@@ -179,6 +179,12 @@ the steps bellow.
    Feature.enable(:repository_push_audit_event)
    ```
 
+## Retention policy
+
+On GitLab.com, Audit Event records become subject to deletion after 400 days, or when your license is downgraded to a tier that does not include access to Audit Events. Data that is subject to deletion will be deleted at GitLab's discretion, possibly without additional notice.
+
+If you require a longer retention period, you should independently archive your Audit Event data, which you can retrieve through the [Audit Events API](../api/audit_events.md).
+
 ## Export to CSV **(PREMIUM ONLY)**
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/1449) in [GitLab Premium](https://about.gitlab.com/pricing/) 13.4.