Outdated documentation for Skeleton3D

**Your Godot version:**
v4.0.2.stable.official [7a0977ce2]

**Issue description:**
Two things are wrong in the `Description` section of the documentation:
- Custom poses don't exist anymore
- "The overall transform of a bone with respect to the skeleton is determined by the following hierarchical order: rest pose, custom pose and pose." - This is the same language as the docs for Godot 3.5. However, in Godot 3.5 the rest pose, custom pose and pose are additive whereas in Godot 4.0 it's just the pose that determines the actual pose of a bone.

The abovementioned problem is repeated in the description for `get_bone_pose`: "Returns the pose transform of the specified bone. Pose is applied on top of the custom pose, which is applied on top the rest pose."

**Custom poses** don't exist, **rest poses** seem to be the default values for **poses** rather than part of an additive chain.

**URL to the documentation page:**
https://docs.godotengine.org/en/4.0/classes/class_skeleton3d.html