Three stars. Rated PG-13. 145 minutes. The Chinese contemporary visual artist Ai Weiwei — whose portraits of political dissidents, formed from Lego blocks, are currently on view at the Hirshhorn Museum and Sculpture Garden in Washington, D.C. — has never limited himself to a single medium. Over his career, the 60-year-old has produced powerful sculptures, installations, photographs, videos, even a stream of social-media postings that can be read as both a form of performance art and as political statement. Ai’s heartbreaking new documentary “Human Flow,” on the subject of the global refugee crisis, continues a tradition of making work that is pungent conceptually and aesthetically.