When a documentary is called “The Meaning of Hitler,” there are two things you know off the bat. One is that the film probably won’t live up to that title — and doesn’t have to, because how could it? The other thing you know is that it’s trying for something audacious, placing itself on the high bar of who-was-Adolf-Hitler? meditation. And that’s a good thing, since for all the mystery that still surrounds Hitler we do know a great deal about him, and we want a movie like this one to jolt us with the shock of the new. The author Martin Amis, who’s one of the most compelling people interviewed here, says that if you can expand our knowledge of Hitler by just a millimeter, you’ve done something. We go into “The Meaning of Hitler” craving that millimeter of insight, of intrigue and revelation. And the film provides it. It ruminates on Hitler and the Third Reich in ways that churn up your platitudes.