How can I document root namespace of VB .NET project?

Because root namespace has no source code representation, you need to add your XML comment for the VB .NET root namespace in VSdocman options for that project.

Go to Tools - VSdocman menu or press VSdocman button on standard toolbar. VSdocman dialog appears, select Project Properties pane. Set Miscellanous - Root Namespace page. You can document the root namespace there.