Update Rename Worker docs
### Why
Regarding the [rename worker classes process](https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#renaming-worker-classes), by adding a small bit of further guidance, we can improve engineers' ability to get through this task with more certainty and greater efficiency.
### What
Add to docs
- [ ] type of FF to use
- [ ] where to use the FF
- [ ] brief explanation and/or diagram of why this matters from a deployment perspective
issue