[webrtc-pc] Add direction change as reason for "mute" event

stefhak has just created a new issue for https://github.com/w3c/webrtc-pc:

== Add direction change as reason for "mute" event ==
Section 5.3, description of the `track` attribute has
```When one of the SSRCs for RTP source media streams received by receiver is removed (either due to reception of a BYE or via timeout), the mute event is fired at track.```

in its text. However, https://w3c.github.io/webrtc-pc/#set-description can also lead to the track being muted ("`For each track in muteTracks, set the muted state to the value true.`"). Should we add this reason for firing `mute` to the `track` attribute description in 5.3?

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1737 using your GitHub account