Tell users to avoid magical CloudFlare nonsense in the CDN documentation

Summary:
Fixes T9716. Doesn't go into too much detail, but will hopefully
save some pain.

Test Plan: Read all the wonderful text.

Reviewers: #blessed_committers, epriestley, #blessed_reviewers

Reviewed By: #blessed_committers, epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T9716

Differential Revision: https://secure.phabricator.com/D15738