In the early Eighties, the world witnessed Iron Maiden on a Promethean quest for fire, driven on a soul level to deliver “Run to the Hills” and “The Trooper” to humanity. But within a few years, they were exhausted from constant touring with occasional bickering. A new documentary depicts how bad it got, with singer Bruce Dickinson pleading with manager Rod Smallwood for fewer tour dates, saying, “You can’t restring a voice.” Ultimately, Dickinson and guitarist Adrian Smith both quit for these reasons during the band’s golden years. (Both musicians returned in 1999 with refreshed appreciation, and they’ve remained since then.) In the film, Burning Ambition , which opens theatrically on May 7, Dickinson likens the tour grind to “five years in the golden cage.” In one scene, around his 1993 departure, he muses, “Is it all worth it, this madness?”  His answer today is not rhetorical: “The madness is worth it,” Dickinson, 67, tells Rolling Stone with a chuckle over Zoom from his London home.…