improve cname docs around alias and aname

Discord Message

Author: laikulo Channel: #feedback Link: https://discord.com/channels/1405685085923049482/1405686161791516873/1454350958678245472

cnames doc should probably note that ALIAS and ANAME are server-side DNS features, and not all providers have them, and some do it transparently for illegal CNAMEs by the SOA (They also tend to muck with geodns in bad ways)