Clarify `time-scale` field

[chrisn]

Reading the protocol detail in Appendix A, I noticed that the time-scale field in the audio-encoding-offer message
wasn't clear. What value is expected, and what are the measurement units does it use?

[markafoltz]

The explanation of the time-scale is in the description of how times and durations are represented in remote playback messages:

All times, time ranges, and durations (such as position, duration, and seekable-time-ranges) used above use a common media-time value (see Appendix A) which includes a time scale. This allows time values which work on different time scales to be expressed without loss of precision. The scale is represented in hertz, such as 90000 for 90000 Hz, a common time scale for video.

It agree would be better to pull the description further up because it is also used in audio-encoding-offer, video-encoding-offer, and data-encoding-offer.

[markafoltz]

Closed by #298.