HBO has been on an amazing roll of late with its original documentaries, led by Jinx: The Life and Deaths of Robert Durst. But this new one, by Nick Broomfield, attacks another cold case less successfully, by focusing on the former and current residents of South Central Los Angeles, neighbors of Lonnie Franklin, imprisoned in 2010 on counts of multiple mass murder. Was he the “Grim Sleeper” who killed dozens of local women? This nonfiction film asks the question, and looks for answer