Mark RTCRtpTransceiver.stopped as deprecated (#4514)
Mark RTCRtpTransceiver.stopped as deprecated (#4514)

This has just been deprecated in the specification. Initial content
changes have been made to note this is incoming and to prepare
developers to switch to the new way to detect stopped transceivers.
Further updates will come when the change is implemented in browsers.

Reference:
* https://github.com/w3c/webrtc-pc/pull/2220

Content changes:
* https://developer.mozilla.org/en-US/docs/Web/API/RTCRtpTransceiver/stopped
* https://developer.mozilla.org/en-US/docs/Web/API/RTCRtpTransceiver/stop
chrisdavidmills/browser-compat-data,mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data