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**

Beginne mit xml/System.Net.Http/WinHttpHandler.xml und vergleiche dessen Metadaten zu Applies to mit dem im Issue beschriebenen Verhalten der .NET Core 2.1 API-Seite. Ermittle, ob WinHttpHandler in .NET Core 2.1 verfügbar ist, und aktualisiere anschließend die Dokumentation so, dass sie seine Verfügbarkeit korrekt darstellt und bei Bedarf den Pfad für fehlende Funktionen erklärt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.