only get leaf images that are not tagged

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Updating the flags for --orphans, and docs

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)