The API docs for various animation nodes are pretty empty, yet the
tutorial at
https://docs.godotengine.org/en/latest/tutorials/animation/animation_tree.html
contains some details.
These details should be included in the API docs so looking up a
particular class actually provides some information rather than
requiring the user to hunt for a different tutorial.
This also links the AnimationTree tutorial and demo in the docs.
I've found the TPS demo to be the best resource so far for learning
how to use the AnimationTree. This should be easy to find if someone
looks up the AnimationTree API docs.
Finally, this fixes a param typo in AnimationNodeStateMachine.
(cherry picked from commit 584288a32c)
