Adds note about Blackbox probes

John Jarvisrequested to merge
jarv/health-cdn into master
Part of https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/1273 and https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14982, this adds some information about the recent blackbox probes that were added for the Registry CDN.

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