Add a docstring for JobPhase
Add a docstring for JobPhase

Summary: Not Proust, but accurate to my knowledge and better than nothing.

Test Plan: Human audit

Reviewers: josiah

Reviewed By: josiah

Subscribers: changesbot, mkedia, wwu, cf

Differential Revision: https://tails.corp.dropbox.com/D105402

dropbox/changes,wfxiang08/changes,wfxiang08/changes,bowlofstew/changes,dropbox/changes,bowlofstew/changes,dropbox/changes,bowlofstew/changes,wfxiang08/changes,bowlofstew/changes,dropbox/changes,wfxiang08/changes