Update spec with API for session reconnection

* Updated spec with API for session reconnection, in part:

partial interface NavigatorPresentation {
  Promise<PresentationSession> startSession(DOMString url, DOMString presentationId);
  Promise<PresentationSession> joinSession(DOMString url, DOMString presentationId);
}

* Describe behavior of startSession() and joinSession().
* Update code example to handle both starting and joining presentations.
* Outline a set of algorithms to implement the functions of the Presentation API.
* Other minor fixes/cleanups.