BEIJING — China has deported 20 foreigners from Britain, South Africa and India for watching video clips that advocated terrorism and religious extremism, the official state media reported, while two of the tourists reportedly blamed their detentions on a documentary about Genghis Khan. Xinhua News Agency said late Saturday that the foreigners watched an unspecified documentary in a hotel room and later some of them watched video clips that advocated terrorism. Britain’s Press Association reported that the documentary was a BBC production on the 13th century Mongol warrior Genghis Khan, citing a statement from two of the British tourists, husband and wife Hoosain and Tahira Jacobs.