[RKE] Rollback documentation is lacking clear explanation and example

[dkeightley]

Summary:
The upgrade rollback information could be more explicit, with an example or small list of steps to help avoid ambiguity.

• https://rancher.com/docs/rke/latest/en/upgrades/#rolling-back-the-kubernetes-version

Details:
For example, the one-line statement (below) is not clear. Does this rollback also involve updating the kubernetes_version field in the cluster.yml? If so, this is potentially misleading that a snapshot on it's own will perform the version rollback.

A cluster can be restored back to a snapshot that uses a previous Kubernetes version.