Document why dropped

The .NET Core 2.1 docs in the "Applies to" section state that WinHttpHandler applies only to 2.0, 1.1, 1.0. Likewise, navigating to the .NET Core 2.1 page for WinHttpHandler redirects to the .NET Core 2.0 page with this message:

> The requested page is not available for .NET Core 2.1. You have been redirected to the newest product version this page is available for.

Thus, the docs are saying that WinHttpHandler has been dropped. If that's true, some background about how to deal with the missing feature should be included. Or if WinHttpHandler actually lives on, the 2.1 docs should be fixed to indicate that.

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#### Document Details

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* ID: 10a449ca-bae2-357d-c22e-03c3779187ca
* Version Independent ID: eb88a9b2-c382-2fe3-df46-e70b44cf4818
* Content: [WinHttpHandler Class (System.Net.Http)](https://docs.microsoft.com/en-us/dotnet/api/system.net.http.winhttphandler?view=netcore-2.0&viewFallbackFrom=netcore-2.1)
* Content Source: [xml/System.Net.Http/WinHttpHandler.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Http/WinHttpHandler.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**