John Ridley’s documentary “Let It Fall: Los Angeles 1982-1992” takes a long view of the uprising that shook South Los Angeles in April 1992 after four white police officers were acquitted for the beating of black motorist Rodney King. Through archival footage and interviews with retired police officers and residents of South Central, as the area was known at the time, “Let It Fall” explores how 10 years of racial tension and aggressive law enforcement policies exploded into chaos. Ridley was the screenwriter for the Oscar-winning “12 Years a Slave” and creator of the television drama “American Crime.”