Hi Matias! Currently the documentation metadata field is initially set from the schema.usda as you mention. It can, however, be used for “scene documentation” authored by users through the UsdObject APIs.
We’re currently working on making this more distinct, so that the documentation field is solely for scene documentation for end-users, and new additional metadata will be added to have “brief doc”, authored in schema.usda, and accessed by UsdPrimDefinition. This “brief doc” will be similar to the API doc currently in schema.usda, but authored as reference doc for users of the schema classes.