Add documentation on listing migrations by job class

What does this MR do and why?

This MR explains how to use ChatOps to list batched background migrations by a specific job class. This was implemented in gitlab-com/chatops!571 (merged).

References

Screenshots or screen recordings

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.