The idea of a "documentation category" is nice; perhaps @_documentationCategory would clarify the intent here but even as-is the base attribute reads pretty well.
Where I have more trouble is that "ignore underscored" reads as exactly the opposite of its behavior: at face value, it's an attribute for documentation to ignore underscored APIs. Meanwhile, @_documentation(underscored) says at face value that the documentation for the relevant declaration is underscored, which is a synonym for emphasized or marked as important—how surprising it would be for a user that the documentation instead disappears.
To me, it seems it'd be more self-explanatory to write something like @_documentation(hidden) or @_documentation(visible).