Re: [mediacapture-image] Document OverConstrainedError for PTZ in explainer (#229)

Restricting pan, tilt, and zoom constraints in `getUserMedia` to be "ideal" would work.
In this case, the PTZ capability request would be optional, not mandatory.

```js
// [NEW] Does not fail with OverConstrainedError if no PTZ camera. 
await navigator.mediaDevices.getUserMedia({ video: { pan: true } });

// [NEW] Both would be equivalent. 
await navigator.mediaDevices.getUserMedia({ video: { pan: true } });
await navigator.mediaDevices.getUserMedia({ video: { advanced: [{ pan: true }] } });
```

It seems like a bigger change than PTZ according to https://github.com/w3c/mediacapture-main/issues/697


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