Clarify usage of AnimationPlayer with AnimationTree and fill in empty method descriptions

Add clarification to docs to explain that when an `AnimationPlayer`
object is paired with an `AnimationTree`, several properties and
methods exposed by the `AnimationPlayer` class may not work as expected.
The issues occur because an `AnimationTree` runs independently, and
uses its member `AnimationPlayer* player` primarily for its
`Map<StringName, AnimationData> animation_set`.

Added descriptions for `root_motion_track` and `get_root_motion_transform`.

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