As one of the world’s best-known classical guitarists, John Williams has been responsible for both raising the instrument’s profile and expanding its repertoire. Today Williams, 69, has performed and recorded virtually every piece for the standard classical guitar repertoire; he has also championed African, Cuban, and Latin American composers, and collaborated widely. In a recent call from his home in London, he talked about his work and this week’s concert, which marks his 10th appearance for San Francisco Performances.