Chris Willman Senior Music Writer and Chief Music Critic The Black Keys named one of their most popular albums “Brothers,” which seems on the face of it to be a positive statement of purpose and, obviously, fraternalism between the rock duo’s two members, Dan Auerbach and Patrick Carney. But the question remains: what kind of brothers? Like, Oasis’ battling Gallaghers, or some gentler brand of bros? It’s a question fans will think about after seeing “This Is a Film About the Black Keys,” an engaging music documentary that trains a spotlight on some lesser-known, historical tensions between the two as well as what binds them.