Clarify UseSocketsHttpHandler availability in versions (#7307)

Author: karelz

xml/System.Net.Http/HttpClientHandler.xml

@@ -40,9 +40,9 @@ The `HttpClientHandler` class and classes derived from it enable developers to c
 
 ### HttpClientHandler in .NET Core
 
-Starting in .NET Core 2.1, the implementation of the `HttpClientHandler` class was changed to be based on the cross-platform HTTP protocol stack used by the <xref:System.Net.Http.SocketsHttpHandler?displayProperty=nameWithType> class. Prior to .NET Core 2.1, the `HttpClientHandler` class used older HTTP protocol stacks (<xref:System.Net.Http.WinHttpHandler> on Windows and `CurlHandler`, an internal class implemented on top of Linux's native libcurl component, on Linux).
+Starting in .NET Core 2.1, the implementation of the `HttpClientHandler` class was changed to be based on the cross-platform HTTP protocol stack used by the <xref:System.Net.Http.SocketsHttpHandler?displayProperty=nameWithType> class. Prior to .NET Core 2.1, the `HttpClientHandler` class used older HTTP protocol stacks (<xref:System.Net.Http.WinHttpHandler> on Windows and `CurlHandler`, an internal class implemented on top of Linux's native `libcurl` component, on Linux).
 
-You can configure your app to use the older HTTP protocol stacks in one of the following three ways:
+On .NET Core 2.1 - 3.1 only, you can configure your app to use the older HTTP protocol stacks in one of the following three ways:
 
 - Call the <xref:System.AppContext.SetSwitch%2A?displayProperty=nameWithType> method:
 
@@ -66,6 +66,8 @@ You can configure your app to use the older HTTP protocol stacks in one of the f
 
 - Define an environment variable named `DOTNET_SYSTEM_NET_HTTP_USESOCKETSHTTPHANDLER` and set it to either `false` or **0**.
 
+These configuration options are not available starting with .NET 5.
+
 ## Examples
  :::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_Misc/system.net.http.httpclienthandler/cs/source.cs" id="Snippet1":::