[return: attribute] on accessor that returns void

Microsoft's documentation on [Attribute specification](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/attributes#attribute-specification) and Microsoft C# compilers allow a `return` _attribute_target_specifier_ in the following contexts, where ECMA-334 5th edition § 22.3 (Attribute specification) does not allow it:

- `add` accessor of an event
- `remove` accessor of an event
- `set` accessor of a property
- `set` accessor of an indexer

```C#
using System;

public class Class1
{
public event EventHandler Event
{
[return: Demo] add { }
[return: Demo] remove { }
}

public int this[int index]
{
get { return 0; }
[return: Demo] set { }
}

public int Property
{
get { return 0; }
[return: Demo] set { }
}
}

[AttributeUsage(AttributeTargets.ReturnValue)]
public sealed class DemoAttribute : Attribute
{
}
```

Because `return:` attributes are generally allowed on methods that return `void`, this feature does not seem to cause any semantic problems.

The following C# compilers allow these attribute specifications even with `/langversion:ISO-1`:

- Microsoft (R) Visual C# Compiler version 2.10.0.0 (b9fb1610)\
Copyright (C) Microsoft Corporation. All rights reserved.
- Microsoft (R) Visual C# Compiler version 4.8.4084.0\
for C# 5\
Copyright (C) Microsoft Corporation. All rights reserved.
- Microsoft (R) Visual C# 2005 Compiler version 8.00.50727.9149\
for Microsoft (R) Windows (R) 2005 Framework version 2.0.50727\
Copyright (C) Microsoft Corporation 2001-2005. All rights reserved.

I don't know whether this feature has been formally proposed in . Anyway, it has already been implemented and documented, so perhaps it should be standardized.