It may be too much to ask a human hummingbird like Alex Edelman to try to stick to the subject. In “Just for Us,” his three-jokes-per-minute one-man show, he zooms from punchline to punchline almost as fast as he caroms around the stage of the Hudson Theater. (At 34, he’s part of what he calls the overmedicated ADHD generation.) If you haven’t read about his act coming to Broadway, you might assume from his introduction — in which he describes his usual style as “benign silliness” and says this “isn’t Ibsen” — that you are in for a cheerful evening...