Re: [mediacapture-main] Clarify getUserMedia({audio:{deviceId:{exact:<audiooutput_device>}}}) in this specification mandates capability to capture of audio output device - not exclusively microphone input device (#650)

`getDisplayMedia()` does not capture audio at all at Chromium. 

`setSinkId()` captures only microphone, not audio output even where the audio output device `deviceId` is selected.

Do not care if `getDisplayMedia()` or `getUserMedia()` needs to be used to capture audio output. If neither method is intended to capture `"audioouput"` whatsoever that needs to be made clear in the specifications. If `getDisplayMedia()` in fact is capable of capturing audio output, e.g., to "headphones", as Media Capture and Streams specification currently implies by language 

> `audiooutput` | Represents an audio output device; for example a pair of headphones.

though that language is actually intended to apply to `getDisplayMedia()`, a canonical example of capturing audio output only - not audio input from a microphone - needs to be included in the specification.

The use case is very simple: capture audio output of `speechSythesis.speak()` which outputs audio directly from the system (https://github.com/guest271314/SpeechSynthesisRecorder; https://github.com/WICG/speech-api/issues/69).

Or, make it unequivocally clear that neither this parent specification nor any derivative specification is intended to capture audio output, so that alternative, non-standardized approaches, can be implemented.

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