Add USB digital audio documentation

Bug: 16836983
Change-Id: I3dea7054b37fea42c7fa89c1a22ed100e00551fd
diff --git a/src/devices/devices_toc.cs b/src/devices/devices_toc.cs
index b20ea6d..7fd0900 100644
--- a/src/devices/devices_toc.cs
+++ b/src/devices/devices_toc.cs
@@ -49,6 +49,7 @@
           <li><a href="<?cs var:toroot ?>devices/audio_src.html">Sample Rate Conversion</a></li>
           <li><a href="<?cs var:toroot ?>devices/audio_terminology.html">Terminology</a></li>
           <li><a href="<?cs var:toroot ?>devices/audio_debugging.html">Debugging</a></li>
+          <li><a href="<?cs var:toroot ?>devices/audio_usb.html">USB Digital Audio</a></li>
         </ul>
       </li>
       <li><a href="<?cs var:toroot ?>devices/bluetooth.html">Bluetooth</a></li>