Improve "Async return types" docs

[xakep139]

Summary

This PR rephrases a misleading statement regarding an awaiter type - an AsyncMethodBuilderAttribute should be applied on a type that an async method returns, not on a type that GetAwaiter() method returns.

The same requirement is showcased in the spec - https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/classes#15141-general.

As an example:

async CustomTask SomeMethodAsync() { /* ... */ }

// The attribute should be applied here:
[AsyncMethodBuilder(typeof(CustomAsyncMethodBuilder))]
class CustomTask
{
    public CustomAwaiter GetAwaiter() { /* ... */ }
}

class CustomAwaiter : INotifyCompletion
{
    public void OnCompleted(Action continuation) { /* ... */ }
}

internal class CustomAsyncMethodBuilder
{
    // The builder must implement a bunch of methods like (only a few required are listed):
    public static CustomAsyncMethodBuilder Create() { /* ... */ }
    public void SetException(Exception exception) { /* ... */ }
    public void SetResult() { /* ... */ }
    public CustomTask Task { get; }
}

Rigth now the documentation states that an AsyncMethodBuilderAttribute should be applied on a CustomAwaiter type from the example above, which is misleading.

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docs/csharp/asynchronous-programming/async-return-types.md	Async return types (C#)