The ReferenceEqualityComparer.Instance property is incorrectly shown as nullable

https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.referenceequalitycomparer.instance?view=net-5.0:

> ```cs
> public static System.Collections.Generic.ReferenceEqualityComparer? Instance { get; }
> ```

Having this property be nullable makes no sense. Docs tooling bug maybe? The .NET Core source shows that it's not nullable:

https://github.com/dotnet/runtime/blob/v5.0.0-rc.1.20451.14/src/libraries/Common/src/System/Collections/Generic/ReferenceEqualityComparer.cs#L30

```cs
///
/// Gets the singleton instance.
///
public static ReferenceEqualityComparer Instance { get; } = new ReferenceEqualityComparer();
```

https://github.com/dotnet/runtime/blob/v5.0.0-rc.1.20451.14/src/libraries/System.Collections/ref/System.Collections.cs#L418

```cs
public static System.Collections.Generic.ReferenceEqualityComparer Instance { get { throw null; } }
```

Compara la firma nullable de ReferenceEqualityComparer.Instance en la página de la API con las declaraciones en tiempo de ejecución de src/libraries/Common/src/System/Collections/Generic/ReferenceEqualityComparer.cs y src/libraries/System.Collections/ref/System.Collections.cs. Empieza comprobando cómo las herramientas de documentación derivan la firma. La tarea está terminada cuando la referencia de API generada muestra la propiedad como no nullable.

Escrito por el modelo de indexación a partir del texto del issue.