Issues with apex custom domains

Relevant issue https://gitlab.com/gitlab-com/support-forum/issues/569

This should be better documented and fixed in the Details page when adding a new domain. If the new domain is not a subdomain, the following info should show an A record with the IP address. I learned this the hard way when my MX records were broken.

Our current IP that the pages daemon listens to is 104.208.235.32.

Few links:

• https://devcenter.heroku.com/articles/apex-domains
• https://serverfault.com/questions/613829/why-cant-a-cname-record-be-used-at-the-apex-aka-root-of-a-domain
• https://stackoverflow.com/questions/16022324/how-to-setup-dns-for-an-apex-domain-no-www-pointing-to-a-heroku-app

cc @ayufan