https://bugzilla.redhat.com/show_bug.cgi?id=869100 mentioned some
confusion about 'virsh snapshot-list' errors.  Clean up a
misleading error message, and add some documentation.

* tools/virsh-snapshot.c (cmdSnapshotList): Mention --current
rather than --from when appropriate.
* tools/virsh.pod (snapshot-list): Mention that the named starting
point is NOT part of the list except under --tree.