Merge branch 'document-zoekt-user-setting' into 'master'

Document user-level option to disable Zoekt

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/171400



Merged-by: Ashraf Khamis <akhamis@gitlab.com>
Co-authored-by: Katrin Leinweber <kleinweber@gitlab.com>