Watching the 76-year-old Hall of Famer Reggie Jackson sit down for an interview at the beginning of “Reggie” is a reminder of what an understated public profile he has maintained in recent years. He wasn’t just a superstar, after all; he had his own candy bar. So when he explains that “the timing was right” for a documentary, he seems to mean that he’s overdue in telling his story. He doesn’t mean that at all.