What makes "Musk" such a lacerating and important watch, grounded in Alex Gibney’s supremely insightful but user-friendly mastery of the form, is that it takes this flamboyant but in many ways elusive figure, with his tech ambitions and cosmic dreams and malignant megalomania, and puts him all together, letting us see how one chapter of his life led to the next, and how the Musk dream wasn’t always so grandiose.