Who are Sparks? What are they about? That’s exactly what Edgar Wright’s outstanding new documentary “The Sparks Brothers” tries to unpack and demystify. Because Sparks is not only a versatile art-pop band (chiefly brothers, Ron and Russel Mael), but they’re a highly eclectic, trailblazing rock group that’s fairly elusive and mysterious to a lot of people, even music connoisseurs.