Add HTML comment explaining 503 issue

What does this MR do?

These links were identified as problematic in the December 2020 link check:

  public/ee/administration/git_protocol.html:
    [ ERROR ] external_links - broken reference to https://dashboards.gitlab.com/d/pqlQq0xik/git-protocol-versions: 503

  public/ee/administration/integration/terminal.html:
    [ ERROR ] external_links - broken reference to https://varnish-cache.org/docs/4.1/users-guide/vcl-example-websockets.html: 404

The first one is a link to a GitLab resource behind an auth wall. We can either put it in inline code tags or we can put a HTML comment in place. I chose the latter but can be convinced otherwise.

Second one is a valid link. Probably an intermittent failure.

Related issues

Closes #290982 (closed)

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