Improve docs for creating ceph storage

[tvansteenburgh]

See https://discourse.jujucharms.com/t/correct-way-for-adding-ceph-storage-class-to-kubernetes/2691 for the context.

The question illustrates that the steps required are not clear. If you use the default storage pools that are created by the master charm when the relation is established, the k8s storageclasses are created for you. If you create your own pool, you must also create a storageclass for it.

We should update the "Create storage pools" section of https://ubuntu.com/kubernetes/docs/storage to make this more clear.