Messages of the _warning method family inform the sender of a problem, but not as grave as an _error or a _failure. It means the sender's request has been processed, but maybe not as intended.
The most common variant of _warning is _warning_usage. It returns usage syntax of manual commands. It is customary to have a derivate of this for each command, as in _warning_usage_mandate.
Other examples are _warning_unable_reply to tell you are talking to someone who will not be able to reply or _warning_usage_subscribe to show you how you can use the /subscribe command.
_warning_server_shutdown is actually a wrong use of _warning, as you didn't request anything and the event is simply coming your way. It should be a _notice. TODO.