A discussion on the ceph-users list uncovered a bit of uncertainty
about how exclusive locking works:
https://www.mail-archive.com/ceph-users@lists.ceph.com/msg56910.html
Add a bit of background information about exclusive locks, and
cross-reference the documentatation on OpenStack, and on CephX.
Signed-off-by: Florian Haas <florian@citynetwork.eu>
