(By Steven Brown, sbrown@charlotteobserver.com) After three years of polishing, one of Charleston's gems will be back on display during this year's Spoleto Festival USA.The Dock Street Theatre, whose coziness has enhanced Spoleto plays, operas and chamber music, will welcome audiences again after three years of extensive renovation. Naturally, the festival - to run from May 28 to June 13 - is taking some cues from the theater's reopening.