Document the expected types of /get_scene service

[scpeters]

The /get_scene service expects an ignition::msgs::SceneRequest message as the request type and replies with ignition::msgs::Boolean. The SceneRequest message it self contains the name of a service that expects an ignition::msgs::Scene message as the request with no reply type (see delphyne_gui/visualizer/render_widget.cc).

I think the intermediate service could be avoided if the /get_scene service had no request type and replied with a Scene (which is what the ign-gui2 Scene3D plugin expects).

[scpeters]

I just realized some of this is already documented here:

• https://github.com/ToyotaResearchInstitute/delphyne/blob/master/src/backend/replayer.cc#L52-L55