AnimationTree documentation is unclear on how to do some stuff you can do with AnimationPlayer

**Your Godot version:** 3.2.2

**Issue description:** [This PR](https://github.com/godotengine/godot/pull/39234) clarified the fact that, if you have an AnimationTree, certain fields and methods of the linked AnimationPlayer return bogus values. It also added a vague statement to the doc that says: "Playback and transitions should be handled using only the AnimationTree and its constituent AnimationNode(s).".

However it's still totally obscure to me how to achieve things like:
- getting the current animation (equivalent of `AnimationPlayer.current_animation`)
- getting the current animation position (`current_animation_position`)
- getting the current animation length (`current_animation_length`)
- etc. (basically all the list reported in the PR linked above).

It's also unclear how to get a specific AnimationNode from the AnimationTree in order to even start "using" it like the doc is suggesting.

Are there ways to do the stuff listed above? Could the doc be updated to point users to the right direction?

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/stable/classes/class_animationnode.html#class-animationnode