Note effect of SSH expiration enforcement

It may not have been clear to users that enabling SSH expiration
enforcement will cause all expired SSH Keys to stop being usable,
so this adds a brief explanation as warning.

doc/user/admin_area/settings/account_and_limit_settings.md

@@ -191,6 +191,8 @@ You can prevent the use of expired SSH keys with the following steps:
 1. Expand the **Account and limit** section.
 1. Select the **Enforce SSH key expiration** checkbox.
 
+Enforcing SSH key expiration immediately disables all expired SSH keys.
+
 For more information, see the following issue on [SSH key expiration](https://gitlab.com/gitlab-org/gitlab/-/issues/320970).
 
 ## Optional non-enforcement of Personal Access Token expiration **(ULTIMATE SELF)**