Danger comment when using ActiveRecord bulk update methods

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126751



Merged-by: Lin Jen-Shin <jen-shin@gitlab.com>
Approved-by: Lin Jen-Shin <jen-shin@gitlab.com>
Reviewed-by: Imre Farkas <ifarkas@gitlab.com>
Reviewed-by: Lin Jen-Shin <jen-shin@gitlab.com>
Reviewed-by: David Dieulivol <ddieulivol@gitlab.com>
Co-authored-by: Drew Blessing <drew@gitlab.com>