Docs: clarify DNS records for Pages

What does this MR do?

• Clarify that DNS MX records will break if you use a CNAME with Pages
• Clarify that we do not recommend it

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/42957

Author's checklist

• Apply the correct labels and milestone

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)