Document how to purge state created by Flocker

fixes #416
If you want to destroy **all** of the Flocker-created volumes, then "destroying" the flocker "dataset" is actually a simpler way to go.

    zfs destroy -r flocker

will destroy all of the filesystems beneath `flocker`.  It will **not** destroy `flocker` because there is actually no `flocker` dataset.  Instead there's a `flocker` pool - which `zfs destroy` does not touch (you have to use `zpool` if you want to destroy it).