From Mexican Soil to Museum Sanctum; San Antonio Enshrines Nelson Rockefeller's Hobby
New Nelson A Rockefeller Center for Latin American Art at San Antonio Museum of Art contains staggering 2,500-piece collection of folk art assembled by Rockefeller; not only has one of world's largest Latin American folk art collections, but also has extensive galleries devoted to pre-Colombian art, Spanish colonial pieces and modern and contemporary works by Latin artists; will present Latin American art spanning nearly three milleniums, from 1,000 BC to present; photo of curator, Marion Oettinger Jr; other photos (M)