Documenting practice with candidate events

Three changes:
* RTCIceCandidate is now less optional and immutable
* sdpMid and sdpMLineIndex are mandatory for the browser to provide
* the candidate attribute on the icecandidate event is now actually nullable and there is documentation that explains what it means
does this work on an offer with no mid ? I have not thought about this all carefully yet as it is hard to see what has actually changed so it may all be fine - just need to make sure that case is not a problem