How do you include Xml Docs for a class library in a NuGet package?

The problem was that I didn't check "Generate Xml Documentation" for the build configuration I was using. That nuspec is correct.

In .NET Core/Standard you can do this by editing the project XML file, for example:

<PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
</PropertyGroup>

<PropertyGroup>
    <DocumentationFile>bin\$(Configuration)\$(TargetFramework)\$(AssemblyName).xml</DocumentationFile>
</PropertyGroup>

This will output the documentation as an XML file next to your output assembly.

EDIT: As a side note once you enable GenerateDocumentationFile you will probably get lots of warnings on your public methods for not having added full documentation tags. If you want to disable these warnings simply add in the PropertyGroup:

<NoWarn>$(NoWarn);1591</NoWarn>