Improve add_column documentation

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/150666



Merged-by: Alper Akgun <aakgun@gitlab.com>
Approved-by: Dmitry Gruzd <dgruzd@gitlab.com>
Approved-by: Alper Akgun <aakgun@gitlab.com>
Co-authored-by: Maxime Orefice <morefice@gitlab.com>