Don't apply security.require-https to intracluster requests

Summary:
Ref T10784. Currently, if you terminate SSL at a load balancer (very common) and use HTTP beyond that, you have to fiddle with this setting in your premable or a SiteConfig.

On the balance I think this makes stuff much harder to configure without any real security benefit, so don't apply this option to intracluster requests.

Also document a lot of stuff.

Test Plan: Poked around locally but this is hard to test outside of a production cluster, I'll vet it more thoroughly on secure.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10784

Differential Revision: https://secure.phabricator.com/D15696