* 8/ceph.rst: `rm` subcommand removes osd from osdmap, not the cluster.
the latter is more ambiguous in different contexts.
* rados/operations/add-or-rm-osds.rst: add a subsection of "Replacing an
OSD". update the subsection of "Removing the OSD" with "ceph osd
purge" command
* release-notes.rst: link from it to the new subsection in add-or-rm-osds.rst