WebSocket platform support

The [ClientWebSocket Class docs for ASP.NET Core 2.1](https://docs.microsoft.com/en-us/dotnet/api/system.net.websockets.clientwebsocket?view=netcore-2.1) say, "the only public implementations of client and server WebSockets are supported on Windows 8 and Windows Server 2012."

However, [WebSockets support in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/websockets?view=aspnetcore-2.1) says that WebSockets are supported on "any OS that supports ASP.NET Core."

These contradictory statements of support need to be reconciled.

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#### Document Details

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* ID: 47bc11d0-a599-ca40-82cd-953e2cf7160e
* Version Independent ID: 98845ff1-bd60-0192-5708-ff346da6b62c
* Content: [ClientWebSocket Class (System.Net.WebSockets)](https://docs.microsoft.com/en-us/dotnet/api/system.net.websockets.clientwebsocket?view=netcore-2.1)
* Content Source: [xml/System.Net.WebSockets/ClientWebSocket.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.WebSockets/ClientWebSocket.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**

Inizia da xml/System.Net.WebSockets/ClientWebSocket.xml e confronta la relativa formulazione sulle piattaforme con la documentazione collegata di ASP.NET Core WebSockets. Identifica quale affermazione sul supporto è autorevole, aggiorna la documentazione pertinente in modo che entrambe le pagine siano coerenti e verifica che le pagine renderizzate ClientWebSocket e WebSockets non contengano più affermazioni contraddittorie.

Scritto dal modello di indicizzazione a partire dal testo della issue.