Over the past year, there’s been a revival of interest in the legendary rivalry between Larry Bird and Magic Johnson, with two tomes on the subject — Seth Davis’s When March Went Mad, focusing on the pair’s 1979 NCAA showdown, and Jackie MacMullan’s “in-their-own-words” When the Game Was Ours — drawing favorable attention. HBO Sports’ new documentary Magic & Bird: A Courtship of Rivals, premiering Saturday, fills the visual void in the nostalgia, but also works to uncover greater truths about the duo and the individuals, the rivalry and the sport.