Ceph

The documentation for "ceph osd purge" does not contain a lot of
information about the command and its expectations. In
particular, the meaning of what is meant by this message is unclear.

In response to the command "ceph osd purge 1 --yes-i-really-mean-it", we
get:
2018-05-10 15:18:03.444 7f29c0ae2700 2 mon.a@0(leader) e1
send_reply 0x556281707d40 0x5562818cdcc0 mon_command_ack([{"prefix": "osd
purge", "sure": "--yes-i-really-mean-it", "id": 0}]=-16 osd.1 is not
`down`.
v16) v1

...notice that "down" is in quotes, suggesting it has a special meaning, which
is usually reserved for marking processes with a "down" flag.

However, this command actually requires that the OSD process not
be running, and will not succeed on a running OSD merely marked as
"down".

A brief mention that "osd purge" requires the OSD process to not be
running should be added to the documentation. A complete example
of the purge command's usage would be helpful.