[services] Backup - Add comments to the db entities
Summary:
Context - https://phabricator.ashoat.com/D2950#93128
Some comments to clarify things around DB entities.
Test Plan: None - these are just comments
Reviewers: varun, geekbrother, jimpo, palys-swm
Reviewed By: varun, palys-swm
Subscribers: ashoat, palys-swm, Adrian, atul, benschac
Differential Revision: https://phabricator.ashoat.com/D3489