N/A clock rates?

http://ortc.org/wp-content/uploads/2015/10/ortc.html#widl-RTCRtpCodecCapability-clockRate

```
clockRate of type unsigned long,
Codec clock rate expressed in Hertz, null if unset.
```

This should change to something akin to:

```
clockRate of type unsigned long,
Codec clock rate expressed in Hertz. If unset, the codec clock rate is applicable to any clock rate (as for example in RED, RTX).
```
