Don't recommend CNAME for bare domains in pages custom domains
After adding a bare domain (e.g. example.com) for pages, the details say to set your DNS with a CNAME, but that can break MX records so should not be recommended for bare domains. Instead, most users will need to use A records to the IP addresses directly. If you're using DNSimple or AWS or a few others, you might be able to use an ALIAS or other record which does some magic to respond with dynamically updating A records, but these are non-standard. And at any rate, they don't call them CNAMEs.
To be clear, it's fine to recommend CNAMES for FQDNs. e.g. www.example.com.