This two-part “Frontline” documentary is based on the Iran travels of Thomas Erdbrink, longtime Tehran bureau chief for the New York Times, who married an Iranian woman and now considers himself, as he suggests in his narrative, unlikely to want to live anywhere else any time soon. We are quickly persuaded. This determinedly upbeat documentary is filled with cheer—especially the opening section, which appears devoted to family relations. The film’s producers are evidently under the impression that protracted exchanges of arch drivel between the journalist and his wife about who the boss is in the marriage, reminiscences...