allow_domain call frequency

We've implemented a web service to do domain validation. What we're curious about is the performance impact of issuing a web request in the `allow_domain` method? Also how often is this method called? Is it called ONLY just before the Let's Encrypt sequence?

It would seem if there is a cert for a domain in storage that we wouldn't need to call `allow_domain`.

Perhaps a bit more of this type of explanation would be useful in the docs.

Thank you.

Trace where `allow_domain` is called in the Let's Encrypt sequence and how certificate storage is checked. Review the existing documentation and explain the call frequency, web-request performance implications, and whether stored certificates avoid the call; done means these behaviors are documented clearly.

Written by the indexing model from the issue text.