To date, all Java artifacts (Notes.jar, XPages runtime, etc.) have been compiled without parameter info, leading to things like this:
It'd be good to compile that in.
Additionally (or instead of that), Designer should ship with Javadoc associated with Notes.jar and for all of the XPages runtime plugins. IBM published Javadoc for 9.0.1 years ago, but that's pretty out of date, difficult to consume in an IDE, and doesn't cover the non-"Extensibility API" stuff like `javax.faces.*`.