Underlying data transport explanation

Partial fix for Issue #1699

webrtc.html

@@ -8448,7 +8448,9 @@ <h3><dfn>RTCDataChannel</dfn></h3>
       "RTCDataChannelInit">id</a></code>s.</p>
       <p>Each <code><a>RTCDataChannel</a></code> has an associated
       <dfn data-lt="data transport">underlying data transport</dfn> that is
-      used to transport actual data to the other peer. The transport properties
+      used to transport actual data to the other peer. In the case of SCTP
+      data channels utilizing the <code><a>RTCSctpTransport</a></code>, the
+      underlying data transport is the SCTP stream pair. The transport properties
       of the <a>underlying data transport</a>, such as in order delivery
       settings and reliability mode, are configured by the peer as the channel
       is created. The properties of a channel cannot change after the channel