doc: Updated the usage scenario and made a few syntax edits.

fixes: #6308

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Removed --fs-type option and text.

fixes: #6326

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

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