R1:689c98c47c33

Update database.schema doc to include how to upgrade the schema

Summary: plus update the phid section a little

Test Plan: read it, looked good.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1287

Differential Revision: https://secure.phabricator.com/D3629