`ClaimsPrincipal.Current` documentation incorrect for .net core (all versions)

The documentation for `ClaimsPrincipal.Current` says:
> By default, Thread.CurrentPrincipal is returned. You can change this behavior by setting the ClaimsPrincipalSelector property to specify a delegate to be called to determine the current principal.

That is true for the .NET Framework, but is not true for .NET Core or current .NET 5 previews.

The actual behavior for .net core is as follows:
> By default, null is returned. You can change this behavior by setting the ClaimsPrincipalSelector property to specify a delegate to be called to determine the current principal.

This is because `ClaimsPrincipalSelector` defaults to null (in all frameworks, as correctly documented), but in .NET Framework it has an internal fallback method `ClaimsPrincipal.SelectClaimsPrincipal()`, which uses Thread.CurrentPrincipal. That fallback does not exist in .NET Core.