`~defaults` parameters are not documented

[trainman419]

It looks like the capability server supports default providers in

capabilities/src/capabilities/server.py

Line 458 in 5dae913

self.__default_providers[interface] = rospy.get_param('~defaults/' + interface)

, but this parameter isn't described in the documentation on http://docs.ros.org/kinetic/api/capabilities/html/capability_server.html