[services] Add documentation for base reactors
Summary:
Depends on D4003
https://linear.app/comm/issue/ENG-969/add-documentation-for-the-reactor-base-classes
Test Plan: None - there are just comments
Reviewers: ashoat, palys-swm
Reviewed By: ashoat, palys-swm
Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh
Differential Revision: https://phabricator.ashoat.com/D4004