Require `AudioDecoderConfig.description` for channels > 2 with Opus?

I'm surfacing this issue, originally reported against Chromium's implementation.

The following fails on Chromium but works on Firefox

const config = {codec: 'opus', numberOfChannels: 6, sampleRate: 44100};
console.log(await AudioDecoder.isConfigSupported(config)); // {supported: true}

const decoder = new AudioDecoder({ error: console.error, output: console.log });

decoder.configure(config);

If the channel count is greater than 2 we can either:

• have isConfigSupported() return {supported: false} when AudioDecoderConfig.description isn't set
• generate extradata behind the scenes

Is this worth calling out in the spec, or adding a non-normative note?

@padenot, wdyt?