HA: Document that Gitaly depends on the internal API/Unicorn

Because of git hooks that call /internal/post_receive etc, Gitaly nodes need to talk to the internal API unicorn for authentication. For HA documentation, we need to document that a Gitaly node needs to have unicorn running, or the internal API has to be configured appropriately.

I naively thought I could configure an HA environment with just NFS and Gitaly together, but in the end I just used this for the /etc/gitlab/gitlab.rb:

gitaly['enable'] = true
gitlab_rails['enable'] = true
unicorn['enable'] = true