NASHVILLE — I’m backstage at a hallowed music hall, sitting across from a rock star, and he’s lifting his tattooed arm to grab a tissue.How did I get here? And more vitally, why is John Rzeznik here, laying bare some of his most personal moments?Our journey to this moment began two years earlier, when I was writing a profile on Rzeznik’s Goo Goo Dolls bandmate, Robby Takac. I spoke with Rzeznik by phone, and his candor struck me. On his relationship with Takac, with whom he founded the Goo Goo Dolls in Buffalo in the mid-’80s, Rzeznik volunteered, “Our lines of communication were pretty much c