Update Phriction documentation for drafts

Summary: Depends on D19668. Ref T13197. See PHI840. This updates the documentation to describe how drafts work in more detail.

Test Plan: Read documentation.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13197

Differential Revision: https://secure.phabricator.com/D19669