The people usually left in a superstar’s wake — first managers, original band members, childhood friends — are good sources for two things: rare memorabilia and dirt. Unfortunately Prince: Unauthorized provides neither. This 50-minute documentary about Prince’s early years in Minneapolis tries to dissect the man by examining the boy. But though the filmmakers have excavated relatives, mentors, and grainy black-and-white photos of His Royal Badness sporting an outsize Afro, not one offers much insight into what makes this one-man music industry tick. Instead we get the archetypal rock & roll story: Young kid, his genius immediately recognized by a prescient few, picks up a huge record deal, flops once or twice, has an epiphany (in Prince’s case sparked by the release of Lipps, Inc.’s ”Funkytown” in 1980), then rebounds to greater and greater acclaim. ”His career speaks for itself,” says Prince’s cousin and first drummer, Chazz Smith. ”What can I add to it?” Exactly.