DuplexClientBase<TChannel> does not appear in the "Example" for DuplexClientBase<TChannel>

The example at https://docs.microsoft.com/en-us/dotnet/api/system.servicemodel.duplexclientbase-1?view=netframework-4.7.2 shows how to use a "SampleDuplexHelloCallback" class, which presumably itself makes use of DuplexClientBase<TChannel>.

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* ID: a60284ae-8d43-9d2c-be33-34e9bde0f10a
* Version Independent ID: 8195dc1f-7662-c1b4-f423-f908b92b410e
* Content: [DuplexClientBase<TChannel> Class (System.ServiceModel)](https://docs.microsoft.com/en-us/dotnet/api/system.servicemodel.duplexclientbase-1?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev15.query%3FappId%3DDev15IDEF1%26l%3DEN-US%26k%3Dk(System.ServiceModel.DuplexClientBase%601)%3Bk(TargetFrameworkMoniker-.NETFramework%2CVersion%3Dv4.6.2)%3Bk(DevLang-csharp)%26rd%3Dtrue&view=netframework-4.7.2#feedback)
* Content Source: [xml/System.ServiceModel/DuplexClientBase`1.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.ServiceModel/DuplexClientBase`1.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**