Re: [webrtc-pc] Add more detail about how getParameters and setParameters work.

> An attempt to set a read-only parameter in setParameters() results in a rejection of the promise with an InvalidModificationError. Should addTransceiver behave the same way?

Good point. Now throwing errors.

> When will getParameters() return other attributes of RTCRtpParameters such as rtcp or degradationPreference?

Let's see. `degradationPreference` is only valid for a sender, and should return whatever it was last set to.

For the `RTCRtcpParameters`, it's a bit different between a sender and receiver.

**Sender**:

* `cname` should be there from creation time, since there should only be one CNAME per PeerConnection.
* `reducedSize` should be `true` if the remote description contains "a=rtcp-rsize" and the implementation supports it, and `false` otherwise.

**Receiver**:

* `cname` never set, since it's not signaled in SDP?
* `reducedSize` set to `true` if the receiver is prepared to received reduced-size RTCP. Which means either it was offered (in a local offer; still waiting for answer), or it's fully negotiated (in both local and remote description, `signalingState` is "stable").

Does that all sound reasonable?

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