A treatment tells the story and gives an indication of the style and tone of the project. It is written as a narrative, typically one paragraph per scene, and has no dialogue. This is typically the stage before 1st draft of a script.
A documentary treatment is often the final stage of the writing process, as opposed to in drama. It also states or implies the style in which it will be treated/told, specifying: interviews, reconstructions, archival footage, photographs, maps, diagrams, graphics, etc.