Lucy Walker’s documentary “Waste Land’’ is just what the film’s website says it is: “stirring evidence of the transformative power of art and the alchemy of the human spirit.’’ The movie is about a select few pickers — “catadores’’ — employed to salvage recyclable materials at Jardim Gramacho, a massive outdoor garbage dump in Rio de Janeiro. The Brazilian artist Vik Muniz arrives at Gramacho from his home in New York eager to apply his found-material photography to the catadores. You expect to be stirred. You expect transformation. You expect somebody to mention The Human Spirit. You might even expect to roll your eyes. But Walker is careful to present Muniz as self-aware. One of the highlights of the film, aside from the complex exuberance of the catadores themselves, is the way in which Walker chooses to present her famous subject and the indigent objects of his cause.