In the space of 23 years, Rory Kennedy has made 20 feature documentaries that, sadly, have flown beneath both the public and critical radar. Only four of them have pages on Wikipedia and just five have received enough reviews on Rotten Tomatoes to accurately determine critical “freshness.” According to Box Office Mojo, a mere two of the films she’s directed have reported box office takes totaling $689,415. As the youngest child of Ethel and the late Robert F. Kennedy, it’s probably a safe assumption that earning money by making movies isn’t Rory Kennedy’s main priority. On a quality level with her 2014 “Last Days in Vietnam,” Kennedy’s new Netflix production “Downfall: The Case Against Boeing” should be required viewing for every student in every last film school in this country. Far too many documentarians lack full conviction with their material and frequently attempt to make up for it by injecting …