Document how recentlyAudibleChanged and setMuted interact.

Points to document:
1) After calling setMuted(true), recentlyAudibleChanged signals will still be emitted.

2) recentlyAudibleChanged gets emitted for Flash plugin audio as well (probably for all PPAPI plugins)

3) If a page contains 2 videos, and each one is started in sequence, recentlyAudibleChanged gets emitted only once for whichever video generates sound first.
After both videos are stopped, recentlyAudibleChanged gets emitted when the last audio sound is generated.
Put another way, the signal is emitted whenever any kind of sound is generated / or is completely silent within a page, regardless of how many audio streams there might be.

4) Possibly document that spurious signal emissions will happen.
E.g. when sound is stopped, recentlyAudibleChanged gets emitted with a value of "true" first, and then with a value of false.
When audio starts playing, the signal is emitted twice with a value of true.