Add note about HttpClientHandler (#4066)

* add note about httpclienthandler

Author: gewarren

xml/System.Net.Http/HttpClientHandler.xml

@@ -30,13 +30,16 @@
       <format type="text/markdown"><![CDATA[
 
 ## Remarks
- The `HttpClientHandler` class and classes derived from it enable developers to configure a variety of options ranging from proxies to authentication.
+
+The `HttpClientHandler` class and classes derived from it enable developers to configure a variety of options ranging from proxies to authentication.
 
 ### HttpClientHandler in .NET Core
 
-Starting with .NET Core 2.1, the <xref:System.Net.Http.SocketsHttpHandler?displayProperty=nameWithType> class instead of `HttpClientHandler` provides the implementation used by higher-level HTTP networking APIs. You can configure your application to use `HttpClientHandler` instead in any of the following ways:
+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:
 
-- By calling the <xref:System.AppContext.SetSwitch%2A?displayProperty=nameWithType> method as follows:
+- Call the <xref:System.AppContext.SetSwitch%2A?displayProperty=nameWithType> method:
 
   ```csharp
   AppContext.SetSwitch("System.Net.Http.UseSocketsHttpHandler", false);
@@ -46,7 +49,7 @@ Starting with .NET Core 2.1, the <xref:System.Net.Http.SocketsHttpHandler?displa
   AppContext.SetSwitch("System.Net.Http.UseSocketsHttpHandler", False)
   ```
 
-- By defining the `System.Net.Http.UseSocketsHttpHandler` switch in the *.netcore.runtimeconfig.json* configuration file:
+- Define the `System.Net.Http.UseSocketsHttpHandler` switch in the *.netcore.runtimeconfig.json* configuration file:
 
   ```json
   "runtimeOptions": {
@@ -56,7 +59,7 @@ Starting with .NET Core 2.1, the <xref:System.Net.Http.SocketsHttpHandler?displa
   }
   ```
 
-- By defining an environment variable named `DOTNET_SYSTEM_NET_HTTP_USESOCKETSHTTPHANDLER` and setting it to either `false` or 0.
+- Define an environment variable named `DOTNET_SYSTEM_NET_HTTP_USESOCKETSHTTPHANDLER` and set it to either `false` or **0**.
 
 ## Examples
  [!code-csharp[System.Net.Http.HttpClientHandler#1](~/samples/snippets/csharp/VS_Snippets_Misc/system.net.http.httpclienthandler/cs/source.cs#1)]