Add rollback comments for upgrade

[ry4nz]

A request from TOI session:

As @RaunakShah suggested

If the upgrade fails, the cluster is reverted to MKE 3 automatically. If the upgrade passes and the customer wishes to revert their cluster, they would need to uninstall MKE 4 and restore MKE 3 from a backup.

[RaunakShah]

We should add the rollback information as a section in https://mirantis.github.io/mke-docs/docs/migrate-from-mke-3/

But for reverting to MKE 3 after the cluster upgrade was successful, we can consider adding it to https://mirantis.github.io/mke-docs/docs/getting-started/uninstall-cluster/