Returns service discovery items for the component. If the JID is null then this is request for the top level
request. SM may return disco items on the top level, other components should just return it's top level service
discovery item for null node.
node - is a service discovery node for which the request has been made.
jid - is the jid to which the request has been made.
from - is the request sender address. Some service discovery information is only meant for administrators.
a list of service discovery items for this component or the component itself disco item for the top level
from - a request sender address. Some service disco elements are meant to be available only to system
administrarors. The component is responsible to check whether the sender is the component administrator and
return results appropriate.
a list of elements with service discovery features.