I consider it generally good practice to include the XML documentation file generated by the C# build in NuGet packages alongside the DLLs to provide intellisense documentation
For other people who came in with the same problem - and if the accepted answer does not help...
Make sure that you have set the 'Include XML documentation' setting in the build configuration you are using (not just Debug, as by default)
In Visual Studio:
or in csproj:
true