Document new networking API added between preview 7 and preview 8

[aik-jahoda]

DocId	Source File Path	Docs URL
Removed from API recently F:System.Net.HttpVersion.Version30	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net/HttpVersion.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.HttpVersion.Version30?branch=master
Should be removed from docs, there is ongoing discussion about why the doc tool didn't reflect changes in ref assembly P:System.Net.WebHeaderCollection.Item(System.String)	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net/WebHeaderCollection.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.WebHeaderCollection.Item?branch=master
P:System.Net.Http.WinHttpHandler.EnableMultipleHttp2Connections	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Http/WinHttpHandler.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.Http.WinHttpHandler.EnableMultipleHttp2Connections?branch=master
T:System.Net.Security.ServerOptionsSelectionCallback	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Security/ServerOptionsSelectionCallback.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.Security.ServerOptionsSelectionCallback?branch=master
P:System.Net.Security.SslClientHelloInfo.ServerName	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Security/SslClientHelloInfo.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.Security.SslClientHelloInfo.ServerName?branch=master
P:System.Net.Security.SslClientHelloInfo.SslProtocols	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Security/SslClientHelloInfo.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.Security.SslClientHelloInfo.SslProtocols?branch=master
P:System.Net.Security.SslServerAuthenticationOptions.ServerCertificateContext	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Security/SslServerAuthenticationOptions.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.Security.SslServerAuthenticationOptions.ServerCertificateContext?branch=master
M:System.Net.Security.SslStream.AuthenticateAsServerAsync(System.Net.Security.ServerOptionsSelectionCallback,System.Object,System.Threading.CancellationToken)	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Security/SslStream.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.Security.SslStream.AuthenticateAsServerAsync?branch=master
P:System.Net.Security.SslStream.TargetHostName	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Security/SslStream.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.Security.SslStream.TargetHostName?branch=master
M:System.Net.Security.SslStreamCertificateContext.Create(System.Security.Cryptography.X509Certificates.X509Certificate2,System.Security.Cryptography.X509Certificates.X509Certificate2Collection,System.Boolean)	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Security/SslStreamCertificateContext.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.Security.SslStreamCertificateContext.Create?branch=master
T:System.Net.Security.SslStreamCertificateContext	https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Security/SslStreamCertificateContext.xml	https://review.docs.microsoft.com/en-us/dotnet/api/System.Net.Security.SslStreamCertificateContext?branch=master

Sync Http is tracked here: #41732
System.Net.Http.SocketsHttpHandler.ConnectCallback tracked here: dotnet/dotnet-api-docs#4861

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