A thumbnail history of the dysfunctional relationship between the United States and Saudi Arabia, Rick Rowley’s documentary “Kingdom of Silence” deserves watching already for the briskly authoritative way with which it tells that story. But the two nations’ sordid decades-long exchange of oil, weaponry, and silent treatment of human rights abuses is only the backdrop for Rowley’s real story: the arc from idealist to nationalist to exiled crusader of Saudi journalist Jamal Khashoggi, murdered by his government in Istanbul for speaking his mind about the royal family.