man: Ceph is also an object store

Replace

   Ceph distributed file system

with

   Ceph distributed object store and file system

using the same wording as http://ceph.com/ to help reduce
the idea that Ceph is just a file system.

Signed-off-by: Loic Dachary <loic@dachary.org>
man: update man/ from doc/man/8

As explained in admin/manpage-howto.txt

Signed-off-by: Loic Dachary <loic@dachary.org>