doc/man: document ceph-bluestore-tool set-label-key and rm-label-key

The ceph-bluestore-tool man page never mentioned set-label-key, nor its
companion rm-label-key, even though both have been accepted by the tool
for a long time. The -k/--key and -v/--value options they require were
undocumented as well.

Add entries for both commands to the synopsis, the command list and the
"Device labels" section, and describe the two options. Note that the
size, osd_uuid, btime and description keys map onto real label fields
while any other key is kept as free-form metadata, since that
distinction also determines what rm-label-key is able to remove.

Fixes: https://tracker.ceph.com/issues/79711
Signed-off-by: Akshay Nazare <akshaynazare3@gmail.com>