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