Enable Help for Duplex WCF Service

<services>
  <service behaviorConfiguration="ServiceBehavior" name="...">
    <endpoint
      address="" binding="wsDualHttpBinding"
      behaviorConfiguration="DualHttpEndpointBehavior"
      contract="..." />
    <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
  </service>
</services>

<behaviors>
  <serviceBehaviors>
    <behavior name="ServiceBehavior">
      <!-- ... -->
    </behavior>
  </serviceBehaviors>

  <endpointBehaviors>
    <behavior name="DualHttpEndpointBehavior">
      <webHttp helpEnabled="true"/>
    </behavior>
  </endpointBehaviors>
</behaviors>