It's been two months since Texas Monthly's president, Evan Smith, announced he was leaving the magazine to go into the virtual newspaper business as CEO of Texas Tribune. In the October issue of the Monthly, Smith offers his adios: "What I've Learned." And among those myriad lessons is this one: The magazine's audience is "middle-aged" (ooof!) and "elite" (pow!) and therefore not inclined to buy issues whose covers feature American Idol winners from Burleson, even if the story was written by