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.