Clarify Generator construction with required kind

[tonyherre]

Update description of MediaStreamTrackGenerator to be shorter and clearer that kind is now required by removing the descriptions of handling an empty kind.

[dontcallmedom]

@tprice11 could you provide your changes to the index.bs file instead of the (automatically generated) index.html - I'll separately clarify both the set up and the documentation of the repo to make it clearer that this is the preferred way to contribute.

[alvestrand]

Obsoleted by the change from MediaStreanTrackGenerator(kind) to VideoTrackGenerator.