xep_0030 add feature documentation

Documentation stands that feature implented by plugin should be announced via xep_0030 by doing:

def post_init(self):
    BasePlugin.post_init(self)
    self.xmpp['xep_0030'].add_feature("jabber:iq:register")

Doing so doesn't seems to work. Feature is not listed when requested with a disco request. Instead most of the officials plugins are doing it with something like:

def session_bind(self, jid):
    self.xmpp['xep_0030'].add_feature('jabber:iq:last')