docs: clarify Dedicated Availability Zones

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167427



Merged-by: Lysanne Pinto <lpinto@gitlab.com>
Approved-by: Ermia Qasemi <eqasemi@gitlab.com>
Approved-by: Lysanne Pinto <lpinto@gitlab.com>
Co-authored-by: bmckitrick <bmckitrick@gitlab.com>