Merge branch 'document-risk-of-scope-to' into 'master'

Make it clearer that `scope_to` needs an index to work well

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/169532



Merged-by: Krasimir Angelov <kangelov@gitlab.com>
Approved-by: Krasimir Angelov <kangelov@gitlab.com>
Reviewed-by: Krasimir Angelov <kangelov@gitlab.com>
Co-authored-by: Dylan Griffith <dyl.griffith@gmail.com>