Audio is said to be optional but there's one place that implies that it is not

[henbos]

The spec says audio is optional (e.g. [here](https://w3c.github.io/mediacapture-screen-share/#capturing-logical-or-visible-display-surfaces and here), which is very much intentional, but if you follow the getDisplayMedia() steps we find a contradiction:

The provided media MUST include precisely one track of each media type in requestedMediaTypes. The devices chosen MUST be the ones determined by the user. Once selected, the source of a MediaStreamTrack MUST NOT change.

We should clarify that in the cause of audio the audio track must not be included.

[henbos]

@jan-ivar to add a PR

[guest271314]

@henbos

but if you follow the getDisplayMedia() steps we find a contradiction:

Related #140