Lack of Documentation for Animation Tree's Advance Expression feature

Godot 4.2.2

If I'm not mistaken, it looks like there still isn't any documentation for this.

Hopefully I'm not overstepping, but you might consider asking Jesusemora on the forums if you can add [this tutorial they wrote on the system](https://godotforums.org/d/39521-this-is-how-to-use-animationtree-state-machine-transitions-with-expressions) to the documentation as a starting point


