The APIExplorer is a nice way to automatically build a docs page for your WebAPI RESTful services. The documentation is created using reflection and then the IDocumentationProvider tries to find the textual information about your methods and method parameters.
The problem is that the reference implementation shows how the docs could be retrieved from the XML documentation generated during the compilation of a service. If this is not an option for you, you could use this simple provider which uses System.ComponentModel.DataAnnotations.Display attributes for method decoration:
This way the WebAPI controller could be documented as follows:
An advantage of this approach is that the docs are part of the assembly metadata and there is no need for external XML documentation files.