We are planning to document that. The version of DriverStation that was uploaded last week maps the buttons according to the following diagram: https://w3c.github.io/gamepad/standard_gamepad.svg.
The analog values are mapped a little differently than the diagram. We decided that it makes more sense to have the "forward" direction be positive and "backwards" to be negative (which is opposite the default).
Note: the new version works for Linux & Windows.  Mapping for Chromebook will be added soon.