Life is unfair, and the music industry is worse. If there were a rubric to figure out what makes one performer a household name and another just one more name in the liner notes, the history of pop would read much differently. Turning the spotlight on several career backup singers, Morgan Neville’s 20 Feet From Stardom shows, with great warmth and color, what it might sound like. These are voices and personalities every bit as big as Tina’s and Aretha’s but that, through the vagaries of fate more than anything else, never made what Bruce Springsteen calls “the long walk” from the back of the stage to the front. Most are resigned to their roles in the musical ecosystem, content to have sacrificed their own aspirations for the sake of elevating the art itself. Whether that’s noble or a con, Neville never judges. He just lets them sing. And, in a more perfect universe, that would be enough.