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
Oh, so you receive an offer with no mid.  I don't know.  Is there a rule preventing someone from adding a mid to an existing media section?  Reading RFC 5888, I can't see an actual prohibition.  And if we assume that unknown attributes are ignored, we can probably safely require that one be included.