documentation: explain ceph osd reweight vs crush weight

Using the wording from Gregory Farnum at
http://lists.ceph.com/pipermail/ceph-users-ceph.com/2014-June/040961.html

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2536 from dachary/wip-doc-reweight

documentation: explain ceph osd reweight vs crush weight

Reviewed-by: Laurent Guerby <laurent@guerby.net>