doc/rados: rewrite storage device front matter

doc/rados: rewrite storage device front matter

This PR updates the text in the RADOS Guide
(the Ceph Storage Cluster Guide) that appears
at the beginning of the "Storage Devices"
chapter. I did the following:

- rewrote some of the sentences so that
  they read more like written text than like
  spoken language
- added "Ceph Manager" to the list of daemons
  that a Ceph cluster comprises
- that's about it.

Signed-off-by: Zac Dover <zac.dover@gmail.com>