Per online documentation and the commented code in HelpPageConfig.cs, enabling XML documentation adds an XML file to ~App_data/name_of_file.xml. The project build properties are then set to match that path.
However, when publishing, the XML file doesn't seem to publish, which causes a YSOD when accessing the api's help page. I've verified that the file's Build Action is set to Content. I've also tried playing with the Copy to Output Directory with no effect.
Anyway, for the sake of curiosity, I'm wondering if there's a solution that uses their recommended file location or if it's just some kind of obsolete recommendation; otherwise, I suppose I can try moving it to the bin directory.
I'm using the Web API 2.1 Help Page nuget package.

