Errol Morris’ documentary is a tribute to the genre. He gets in Robert McNamara’s face, and while McNamara is a master of careful prose and sneaky evasions, he is also wonderfully blunt about lessons learned in Vietnam and about the conduct of war and diplomacy. McNamara, defense secretary under JFK and World Bank chief thereafter, waxes loquacious and sentimental. Morris (“The Thin Blue Line,” “Gates of Heaven”) dubs in historical images and has a haunting, flute-driven score by Philip Glass. McNamara’s 11 lessons are poignant because they’re sensible and largely unheeded or ignored.