Clarify UseSocketsHttpHandler version availability (dotnet#5211)

Author: karelz

xml/System.Net.Http/SocketsHttpHandler.xml

@@ -34,7 +34,7 @@ Starting with .NET Core 2.1, the `SocketsHttpHandler` class provides the impleme
 
 - Consistent behavior across all .NET platforms.
 
-If this change is undesirable, you can configure your application to use the older <xref:System.Net.Http.HttpClientHandler?displayProperty=nameWithType> class instead in a number of ways:
+If this change is undesirable and you are on .NET Core 2.1-3.1, you can configure your application to use the older <xref:System.Net.Http.HttpClientHandler?displayProperty=nameWithType> class instead in a number of ways:
 
 - By calling the <xref:System.AppContext.SetSwitch%2A?displayProperty=nameWithType> method as follows:
 
@@ -57,6 +57,8 @@ If this change is undesirable, you can configure your application to use the old
 
 - By defining an environment variable named `DOTNET_SYSTEM_NET_HTTP_USESOCKETSHTTPHANDLER` and setting it to either `false` or 0.
 
+These configuration options are not available starting with .NET 5.
+
          ]]></format>
     </remarks>
   </Docs>