[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