[webrtc-svc] Clarify behavior of getParameters() (#68)

aboba has just created a new issue for https://github.com/w3c/webrtc-svc:

== Clarify behavior of getParameters() ==
It would be useful to clarify the behavior of getParameters() before and after negotiation has completed. Specifically: 

- What is returned if `scalabilityMode` was not set. 
- What is returned if an attempt was made to set `scalabilityMode`, but the attempt failed.
- What is returned if `setCodecPreferences()` is used to change the preferred codec.

Please view or discuss this issue at https://github.com/w3c/webrtc-svc/issues/68 using your GitHub account


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