Update src/rgw/rgw_admin.cc

Improved the usage message.
mon: Monitor: track timecheck round state and report on health

Fixes: #3854

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: timecheck: only output report to dout once

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>