Document multi-factor authentication
Summary: Ref T4398. This has a few lies (in the sense of "features that don't work yet") but should describe behavior accurately after a few more patches.
Test Plan: Read it.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T4398
Differential Revision: https://secure.phabricator.com/D8910