docs: Documented DHU's --adb <port> flag

See first comment for doc stage location.

bug:23721209
Change-Id: If4ce6e72cce555b72176a77c9b3f2d552b71f54e

docs/html/training/auto/start/index.jd

@@ -252,9 +252,13 @@ $ ./desktop-head-unit</pre>
 
     <p>
       By default, the head unit server connects over port 5277. To override the host or port
-      (for example, to forward over SSH), use the <code>--adb</code> flag.
+      (for example, to forward over SSH), use the
+      <code>desktop-head-unit --adb &lt;[localhost:]port&gt;</code> flag, as in
+      the following example:
     </p>
 
+    <pre class="no-pretty-print">$ ./desktop-head-unit --adb 5999</pre>
+
   </li>
 </ol>