Clarify simulcast envelope is a sender property

[jan-ivar]

The spec says: "An RTCRtpTransceiver's simulcast envelope is established in the first successful negotiation that involves it sending simulcast instead of unicast, and includes the maximum number of simulcast streams that can be sent, as well as the ordering of its encodings."

Since simulcast is entirely about sending, it'd be clearer to call it a property of RTCRtpSender, which is the "it" in "its encodings".