Add headings for SEP `scan` action behavior details

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/157700



Merged-by: Russell Dickenson <rdickenson@gitlab.com>
Approved-by: Grant Hickman <ghickman@gitlab.com>
Approved-by: Russell Dickenson <rdickenson@gitlab.com>
Co-authored-by: Connor Gilbert <cgilbert@gitlab.com>