[services] Add comments about indexes

Summary:
Raised here https://phabricator.ashoat.com/D3087#86770

Adding comments in places where database indexes are needed.

Test Plan: None, these are just comments

Reviewers: palys-swm, varun, atul, geekbrother, jimpo, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, benschac

Differential Revision: https://phabricator.ashoat.com/D3420