Why should you see a documentary about backup singers? One look at the electrifying 20 Feet From Stardom and you’ll have the answer, that is, after you stop cheering. Lend your eyes and ears to the likes of Darlene Love, Merry Clayton, Lisa Fischer and Tata Vega, and get ready to revise your definition of what a star is. Director Morgan Neville and producer Gil Friesen (the A&M Records exec who died last year) have rightly gone into the shadows to shine a light on sheer perfection. Mick Jagger, Bruce Springsteen, Stevie Wonder, Bette Midler and Sting all bear witness and theorize about why that 20-foot leap to the star spot didn’t happen for these unsung divas. Sometimes it’s racism; sometimes bum luck; sometimes it’s producer Phil Spector putting Love’s voice in another singer’s mouth. You watch. You hear the gospel spoken in the voices of these women. And you marvel.