Update doc for WebTransport.

- Add new API changes.
- Add examples.

Author: jianjunz

docs/design/webtransport.md

@@ -10,7 +10,39 @@ Following APIs will be changed to support QuicTransport.
 
 - `LocalStream` can be constructed with a `WritableStream`.
 - `RemoteStream` can be constructed with a `ReadableStream`.
+- `ConferenceClient` has new method `createSendStream` which returns a `LocalStream` with a unidirectional stream in it.
+- `StreamSourceInfo` has a new bool property `data`.
+
 
 ## Internal Changes
 
 JavaScript SDK creates a QuicTransport with a QUIC agent when QUIC agent is enabled at server side, and WebTransport is supported at client side. When app publishes or subscribes a data stream, a new QuicStream is created.
+
+## Examples
+
+### Send data to a conference
+
+```
+const sendStream = conferenceClient.createSendStream();
+const publication = await conferenceClient.publish(sendStream);
+sendStream.stream.write(somethingToWrite);
+```
+
+### Receive data from a conference
+
+```
+conferenceClient.addEventListener('streamadded', async (event) => {
+  if (event.stream.source.data) {  // Data stream.
+    const subscription = await conference.subscribe(event.stream);
+    const reader = subscription.stream.readable.getReader();
+    while (true) {
+      const {value, done} = await reader.read();
+      if (done) {
+        // Stream ends.
+        break;
+      }
+      ProcessData(value);
+    }
+  }
+});
+```