ConfiguredCancelableAsyncEnumerable.WithCancellation needs an example

An example of how to use this method would be appreciated. For example, suppose I have a class that streams out messages in a .NET Core 3.0 C# 8.0 beta project:

```
public class StreamGenerator
{
public async IAsyncEnumerable GetMessageStream()
{
for (int i = 0; i < 5; i++)
{
await Task.Delay(500);
yield return $"Message {i + 1}";
}
}
}
```

Then I can use it like so:

```
var incommingMessages = new StreamGenerator();
await foreach (var message in incommingMessages.GetMessageStream())
{
Console.WriteLine(message);
}
```

Now suppose I want cancellation support. Just to sanity check let's cancel the token even before we start streaming in messages:

```
var cts = new CancellationTokenSource();
cts.Cancel();
var incommingMessages = new StreamGenerator();
await foreach (var message in incommingMessages.GetMessageStream().WithCancellation(cts.Token))
{
Console.WriteLine(message);
}
```

The expected behavior would be that an OperationCanceledException occurs, but this is not the case. The await foreach loop runs as if no cancellation had been requested.

Could you provide an example of how to use this extension method?

---
#### Document Details

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* ID: 421b581f-d44b-1f32-f211-fd261e505c42
* Version Independent ID: 8bd92f2e-1e49-ec33-fd56-0187807d7604
* Content: [TaskExtensions.WithCancellation(IAsyncEnumerable<T>, CancellationToken) Method (System.Threading.Tasks)](https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.taskextensions.withcancellation?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev15.query%3FappId%3DDev15IDEF1%26l%3DEN-US%26k%3Dk(System.Threading.Tasks.TaskExtensions.WithCancellation``1)%3Bk(DevLang-csharp)%26rd%3Dtrue)
* Content Source: [xml/System.Threading.Tasks/TaskExtensions.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Threading.Tasks/TaskExtensions.xml)
* Product: **dotnet-api**
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* Microsoft Alias: **dotnetcontent**