Issue #153: Clarification in Section 7.3.2

A question has arisen as to how RTCRtpReceiver.getCapabilities(kind) should behave with respect to codecs that have no intrinsic kind (such as RTX or RED).   For example, if we assume that RTX and RED can be used with kind === 'audio' or 'video', should these codecs be returned regardless of the value of kind?  Also, should RTX and/or RED be returned as a codec capability at all?

Below is some proposed text for Section 7.3.2 to describe the desired behavior of RTCRtpReceiver.getCapabilities:

getCapabilities, static
Obtain the receiver capabilities, based on kind. If kind is omitted or set to "", then all capabilities are returned. To avoid confusion, getCapabilities(kind) should return codecs with a matching intrinsic kind value, as well as codecs with no intrinsic kind, (such as the retransmission payload [RFC4588] and redundancy [RFC2198]), with the value of RTCRtpCapabilities.RTCRtpCodecCapability[i].kind set to the value of kind passed as an argument to RTCRtpReceiver.getCapabilities(kind).