In “A River Changes Course,” lawyer-turned-filmmaker Kalyanee Mam’s first feature-length documentary, Ms. Mam follows three Cambodian families in remote villages who find the modern world closing in on their traditional ways of life. One young woman, Khieu Mok, leaves her village so she can work in a garment factory; she sends the vast majority of her wages back home so her mother can pay off bank debts. Another teenage boy, Sari Math, goes to work…