Warn that k3s-uninstall.sh deletes local-path-provisioner files

[benwaffle]

Is your feature request related to a problem? Please describe.
When I uninstalled k3s, I assumed it would leave around local path PVs, based on the warning on this page: https://docs.k3s.io/installation/uninstall. I didn't loose any important data, but it was surprising.

Describe the solution you'd like
Leave the files in /var/lib/rancher/k3s/storage/* around and tell the user about it.

[brandond]

We could perhaps amend the warning on that page to make it clear that external Kubernetes storage does not include local-path-provisioner volumes, as these are not external.

We're unlikely to modify the uninstall script to leave additional things behind, this is the sort of "personal preference" change that while you might consider helpful, someone else would open an issue to complain about if we did do it that way.