Specify that Certbot requires a publicly available domain/server

Without a public domain, no CA will issue you a publicly trusted certificate. Without that domain pointing to a publicly accessible server, you must use the DNS challenge to issue certificates.

These requirements weren't clear to at least one user at https://github.com/certbot/certbot/issues/7235 though so we should try to do a better job specifying them (in a way that doesn't confuse more users). Some ideas on how to do this are in the above GH issue.