Merge pull request #2760 from jan-ivar/simulcastenvelope

Clarify simulcast envelope is determined by negotiation.

webrtc.html

@@ -11024,13 +11024,17 @@ <h3>
           </h3>
           <p data-test="simulcast/basic.https.html">
             Simulcast functionality is provided via the
-            {{RTCPeerConnection/addTransceiver}} method of the
-            {{RTCPeerConnection}} object and the {{RTCRtpSender/setParameters}}
-            method of the {{RTCRtpSender}} object.
+            {{RTCPeerConnection/addTransceiver}} method with a
+            {{RTCRtpTransceiverInit/sendEncodings}} argument or the
+            {{RTCPeerConnection/setRemoteDescription}} method with a remote
+            offer to receive simulcast, both on the {{RTCPeerConnection}} object,
+            and the {{RTCRtpSender/setParameters}}
+            method on the {{RTCRtpSender}} object.
           </p>
           <p class="needs-test">
-            The {{RTCPeerConnection/addTransceiver}} method establishes the
-            <dfn>simulcast envelope</dfn> which includes the maximum number of
+            An {{RTCRtpTransceiver}}'s <dfn>simulcast envelope</dfn> is
+            established through the first negotiation it participates in, and
+            includes the maximum number of
             simulcast streams that can be sent, as well as the ordering of the
             {{RTCRtpSendParameters/encodings}}. While characteristics of
             individual simulcast streams can be modified using the
@@ -11043,10 +11047,9 @@ <h3>
             simulcast.
           </p>
           <p class="needs-test">
-            Another implication is that the answerer cannot set the [=
-            simulcast envelope =] directly. Upon calling the
-            {{RTCPeerConnection/setRemoteDescription}} method of the
-            {{RTCPeerConnection}} object, the [= simulcast envelope =] is
+            An exception is when the user agent is the answerer. Upon calling the
+            {{RTCPeerConnection/setRemoteDescription}} method with a remote
+            offer to receive simulcast, the [= simulcast envelope =] is
             configured on the {{RTCRtpTransceiver}} to contain the layers
             described by the specified session description. Once the
             envelope is determined, layers cannot be removed. They can be