Working documents are documents that reflect the fact that Garage is a software that evolves quickly. They are a way to communicate our ideas, our changes, and so on before or while we are implementing them in Garage. If you like to live on the edge, it could also serve as a documentation of our next features to be released.
Ideally, once the feature/patch has been merged, the working document should serve as a source to update the rest of the documentation and then be removed.