[webrtc-pc] Pull Request: move "set a configuration" algorithm

fippo has just submitted a new pull request for https://github.com/w3c/webrtc-pc:

== move "set a configuration" algorithm ==
moves the algorithm to set a configuration to the operations section.
Also mentions the [[configuration] internal slot in the constructor.

fixes #1200

See https://github.com/w3c/webrtc-pc/pull/1233