Sam Pollard’s “MLK/FBI” follows the dirty war that America’s FBI declared on civil rights figurehead Martin Luther King, a vendetta that began in the 50s and ended with his assassination in 1968, inspired by recent revelations (as well as credible long-held suspicions), and backed up by declassified secret government documents. Documentary festival IDFA, which runs until Dec. 6, selected the title in their Masters section.