Caitlyn Jenner is best known as a tabloid celebrity and family member of the Kardashian clan who publicly transitioned as a transgender woman. But “Untold: Caitlyn Jenner” — an episode of the new Netflix documentary series “Untold” that highlights underappreciated stories from sports — illuminates the events that brought Jenner her first taste of international recognition; namely, her gold medal win as a decathlon athlete in the 1976 Montreal Olympics.