Document resizing of Kubernetes storage volumes

[a-robinson]

As of Kubernetes 1.11, the ability to resize persistent volume claims is on by default in the major cloud providers. Given that our configuration files default to such a small disk size, I think this is worth including a small section on in our Kubernetes docs.

Upstream documentation: https://kubernetes.io/docs/concepts/storage/persistent-volumes/#expanding-persistent-volumes-claims

[jseldess]

Merged into #5459.