Document how to do a rollback via Brupop

**Issue or Feature Request:**

We should document how to do a rollback via Brupop.

In a scenario where someone wants to rollback an update that has been made automatically, there should be a way to have brupop (or really, the `apiclient` under the hood, coordinated via brupop) to perform a rollback.

It may be as simple as suggesting the following setting on each host:

```
apiclient set settings.updates.version-lock="v1.12.0"
```