Doc updates for Secure *_DISABLED variables

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/117920



Merged-by: Russell Dickenson <rdickenson@gitlab.com>
Approved-by: Russell Dickenson <rdickenson@gitlab.com>
Reviewed-by: rossfuhrman <rfuhrman@gitlab.com>
Co-authored-by: Ross Fuhrman <rfuhrman@gitlab.com>