Add code snippet displaying the behavior of trailing headers in an HTTP response when calling requests with HttpCompletionOption.ResponseHeadersRead

In System.Net.Http.HttpResponseMessage, new property TrailingHeaders returns an empty collection for this corner case. This needs to be documented and a good idea of showing how this works is by adding a snippet of it.

This needs to wait for #2733, then the sample can be added to [dotnet/samples](https://github.com/dotnet/samples).