Update explainer with video conferencing actions

[chrisn]

This issue is to track updating the Media Session use cases and explainer after adding video conferencing actions.

There's a draft explainer in this Gist.

[yuhonglin]

Interesting proposal! Just a few questions:

1. Should we propose some interface like "setCurrentSpeaker". Then the UI can update who is currently speaking in the video conference. This is more suitable than the current "artist" field for video conferencing.
2. Should we have an interface to enable the web app to designate that a particular session is a "conference session" rather than a normal video/audio playing session? For example, something like "setSessionType"?

[youennf]

Can we close this issue given the recent work related to the actions and the existing examples in the spec?