Document WinHttpHandler trailing header feature

[antonfirsov]

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 Use with caution and only in well justified compatibility scenarios, since the property is obsolete

[alexchro93]

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.

hi @antonfirsov, is there any documentation out there currently that says which versions of Windows support this feature? I'm fairly certain most generally available builds of Windows 11 and Server 2022 support trailing headers, but I'm interested to hear if it was ever backported to a recent release of Windows 10 or Server 2019?

I'd run a test to check but I don't currently have access to a build of Windows 10 or Server 2019.