Include XML Comments in NuGet Package

[abergs]

I think this might got lost in the most recent package configuration.

It's currently missing:

It's as simple as adding <GenerateDocumentationFile>true</GenerateDocumentationFile> to the csproj but we get a lot of warnings that could ignore completely, or we should ignore selectively for externallibs. Not sure how to effectively ignore them "partly". Perhaps @MeikTranel knows? Otherwise I'll just ignore them globally.