Improve definition/guidelines of capture mute and/or failure

[youennf]

As was discussed recently, we can observe some differences in the way browsers use muted and ended state for capture tracks.

For instance, Safari will fail capture if it is not receiving any audio or video frame after some time has elapsed.
Apparently, Chrome will instead mute the track (indefinitely?).
I am not sure what Firefox is doing.

It would help documenting these differences and see how much we can help web developers life in that area.

[vr000m]

We recently reported an issue when Chrome triggers a track is muted when all video element srcObjects referring to it are set to null. While FF does not. This causes abstractions that catch mute to behave differently which eventually leads to wrong guidance to the end-user (https://bugs.chromium.org/p/chromium/issues/detail?id=1500277#c10)