If there were a hall of fame for pivotal 20th century moments in music, the recently unearthed documentary “The Capitol Session ‘73” should be inducted. The film captures Bob Marley at a career crossroads, a period when he was transitioning into an international star from his decade-long first-among-equals era in the Wailers. In addition, it provides a rare glimpse of three founding fathers of reggae — Marley, Peter Tosh and Joe Higgs (filling in for a fourth Jamaican giant, Bunny Wailer) — sharing the stage for one of the last times as the art form they helped create was starting to break through worldwide. The original Wailers were on the verge of breaking up, but reggae's next, biggest chapter was just beginning.