doc: Fixed usage typo.

fixes: #4923

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added glossary.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added glossary to TOC.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
Merge git://github.com/ceph/ceph