Add docstrings and comments about snapshots
Add docstrings and comments about snapshots

Summary: Some things were a little unclear. Hope I got the details right.

Test Plan: It's comments only. Unit tests pass.

Reviewers: haoyi

Reviewed By: haoyi

Subscribers: changesbot, mkedia, wwu, jhance

Differential Revision: https://tails.corp.dropbox.com/D111459

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