[services] Document base reactor

Summary:
Depends on D4002

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/D4003