[mediacapture-fromelement] Pull Request: Don't run the steps for MediaStreamTrack.stop() when ending a track

Pehrsons has just submitted a new pull request for https://github.com/w3c/mediacapture-fromelement:

== Don't run the steps for MediaStreamTrack.stop() when ending a track ==
Ending a track for reasons other than stop() is well-defined behavior in
mediacapture-main. To still explicitly mention what should be done, a
reference to the section containing these steps is added to a note.

Fixes #77.

See https://github.com/w3c/mediacapture-fromelement/pull/81