Morgan Neville’s soulfully doting documentary pays tribute to the background singers who’ve been so essential, albeit anonymously, to rock-n-roll history. Aside from their stunning and spine-tingling talents, these great ladies — including Merry Clayton, Lisa Fischer, Claudia Lennear, Darlene Love, and Táta Vega, among many others — seem impressively magnanimous about it being very much a history of white men wanting their own sound to seem periodically more female and more black. Neville whips up a terrific medley of performance footage and generous interviews in which some of the women discuss the deeper role of music in their lives and the vicissitudes of their attempts at solo careers. (Some of the men weigh in, too.) Meanwhile the music speaks for itself. One of many excellent examples explored here: the long-running cultural current that is the great ghostly hook in the Rolling Stones’ “Gimme Shelter,” without which that song would be unthinkable.