API reference doesn't show members for .Net Standard types not referenced from NETStandard.Library

For types that have .Net Standard packages, but are not part of `NETStandard.Library`, it seems the new API reference behaves as if those types did not have .Net Standard versions at all (i.e. it hides all their members).

For example, this happens for [`XmlNode`](https://docs.microsoft.com/en-us/dotnet/api/system.xml.xmlnode?view=netstandard-1.6) (the URL says `netstandard-1.6`, but it seems it changes to `netframework-4.7` on its own, so you have to use the dropdown to see it), which exists in the `System.Xml.XmlDocument` package, which has a version for .Net Standard 1.3.

I think for such types, their .Net Standard version should be shown.

https://apisof.net/ has a very similar issue (https://github.com/terrajobst/apisof.net/issues/2), which could be related.