We should document the new WinHttpHandler trailing header management feature (dotnet/runtime#44778) as impemented in dotnet/runtime#48704.

I recommend to add the docs to the Remarks section.

Key points:

• Currently works only on Windows Manganese (Windows Server for Fe/21H1, aka Windows Server 2022 Build>=19622 if I get it right), but it's under backport, we need to collect proper version info.
  • No trailers are reported if the OS does not support WINHTTP_QUERY_FLAG_TRAILERS.
• On netstandard2.1+ targets, trailers are added to HttpResponseMessage.TrailingHeaders
• On .NET Framework, trailers are added to HttpRequestMessage.Properties["__ResponseTrailers"]. Use with caution and only in well justified compatibility scenarios, since the property is obsolete