overlayfs: more helpful output when not supported

based on https://github.com/docker/docker/pull/7619#discussion_r20385086

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>
overlayfs: add --storage-driver doc

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Merge pull request #9223 from vbatts/vbatts-overlay_notfound

overlayfs: more helpful output when not supported