The documentation you read is meant for low-level use (mostly for how to modify the Spine runtime for your project's needs)
If you're just starting out with Spine-Unity, this documentation is more appropriate: http://esotericsoftware.com/spine-unity-documentation
That said, playing animations backwards is not supported by AnimationState. It always assumes that time moves forwards and that's how it tracks when to play next animations in the queue.
So for playing backwards, you either need to manually Apply the animation yourself, backwards; partly mimicking what AnimationState does for you, at least during those periods of time when you need it.
Or you need to modify AnimationState to include that specialized behavior in its system.