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

beaufortfrancois has just created a new issue for https://github.com/w3c/mediacapture-image:

== Document OverConstrainedError for PTZ in explainer ==
As discussed at https://github.com/w3c/mediacapture-main/issues/697, the resolution to the fingerprinting/capabilities query in `getUserMedia()` in general seems to be to keep things as they are. It means that pan/tilt/zoom properties will be three more properties among the many that `getUserMedia()` already exposes.

In other words, `getUserMedia()` will reject with an `OverConstrainedError` if camera does not support PTZ and PTZ constraints are required without user prompt.

I believe we should document this in the upcoming privacy section of the PTZ explainer.

@riju @reillyeon @eehakkin @andypaicu


Please view or discuss this issue at https://github.com/w3c/mediacapture-image/issues/229 using your GitHub account