doc: Explain OSD versus OSD daemon

I found the use of OSD confusing, as I was reading the
documents. Sometimes, it refers to Object Storage Device, sometimes to
OSD daemon.

I modified the text in a couple of places which I think makes the uses
of OSD more clear.

Please tell me whether this is really more clear, and correct.
Signed-off-by: Kevin Dalley <kevin@kelphead.org>