Document once-off keeps for gitlab-housekeeper

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/143876



Merged-by: Manoj M J <mmj@gitlab.com>
Approved-by: Tiger Watson <twatson@gitlab.com>
Approved-by: Manoj M J <mmj@gitlab.com>
Co-authored-by: Dylan Griffith <dyl.griffith@gmail.com>