Merge branch 'exact-code-search' into 'master'

Improve exact code and advanced search docs

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167580



Merged-by: Dmitry Gruzd <dgruzd@gitlab.com>
Approved-by: Dmitry Gruzd <dgruzd@gitlab.com>
Reviewed-by: Dmitry Gruzd <dgruzd@gitlab.com>
Co-authored-by: Ashraf Khamis <akhamis@gitlab.com>