Docs feedback: Insufficient Omnibus container registry documentation

Link the doc and describe what is wrong with it. https://docs.gitlab.com/ce/administration/container_registry.html

I was unable to simply define registry_external_url per the documentation and have everything work. I may be a somewhat unique situation, as I'm running behind a Traefik reverse proxy with SSL offloaded there, but I had to set several other properties to get my instance to play nicely:

registry_external_url "{{ registry_url }}"
gitlab_rails['registry_enabled'] = true
gitlab_rails['registry_host'] = "{{ devtools_registry_host }}" # This failed to default to the right value

gitlab_rails['registry_api_url'] = "http://localhost:5000" # This failed to default to the right value

registry['enable'] = true # Note: the key is enable, not enabled, using enabled doesn't work
registry['token_realm'] = "{{ gitlab_url }}" # This is undocumented
registry['registry_http_addr'] = "localhost:5000" # This failed to default to the right value

registry_nginx['listen_port'] = 80 # Required for offloaded ssl
registry_nginx['listen_https'] = false # Required for offloaded ssl