Document consolidated form of object storage for ci_secure_files

Closes #473009

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/160411



Merged-by: Evan Read <eread@gitlab.com>
Approved-by: Evan Read <eread@gitlab.com>
Co-authored-by: Achilleas Pipinellis <axil@gitlab.com>