Error handling information needed

The "ReceiveAsync" method does throw exceptions like WebSocketException but it is not mentioned in the document.
Also, after ReceiveAsync throws an exception saying that the remote party closed the connection without closing handshake, the State property of the WebSocket is still Open. Examples on error and closing handling are needed.

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* ID: f3e9d105-0964-0a00-068c-c4160e620ce5
* Version Independent ID: 492a8ab2-9f42-aedc-3349-bc5056316874
* Content: [WebSocket.ReceiveAsync Method (System.Net.WebSockets)](https://docs.microsoft.com/zh-cn/dotnet/api/system.net.websockets.websocket.receiveasync?view=netcore-2.1#System_Net_WebSockets_WebSocket_ReceiveAsync_System_Memory_System_Byte__System_Threading_CancellationToken_)
* Content Source: [xml/System.Net.WebSockets/WebSocket.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.WebSockets/WebSocket.xml)
* Product: **.net**
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