Add 'WinRTManagedOnlyTypeDetails' type

[Sergio0694]

This PR adds the WinRTManagedOnlyTypeDetails, which can be used with WinRTExposedTypeAttribute to correctly suppress warnings from the AOT generator and also suppress the codegen for a type (regardless of whether it's marked as partial). I also added a bunch of XML docs for all of those types involved here, and made WinRTExposedTypeAttribute visible.