Clarify usage of AnimationPlayer with AnimationTree and fill empty doc descriptions

[mrushyendra]

Updated class documentation for AnimationTree:

1. Added missing descriptions for root_motion_track and get_root_motion_transform()

2. Clarified usage of AnimationPlayer when paired with an AnimationTree. The behavior is described in more detail in Correctness of AnimationPlayer's methods is affected when paired with an AnimationTree godot-docs#3610. Essentially, when an AnimationPlayer object is paired with an AnimationTree, several properties and methods exposed by the AnimationPlayer class will not work as expected. Some of these include:

current_animation
assigned_animation
current_animation_position
current_animation_length
playback_default_blend_time
playback_active
playback_speed

as well as their associated setters and getters, and a couple of other methods. This occurs because an AnimationTree uses its AnimationPlayer* player primarily for its Map<StringName, AnimationData> animation_set.

AnimationTree::_process_graph() calls AnimationTree::_update_caches() to get the animation_set from the player and populate its track_cache, instead of calling AnimationPlayer::play() directly. Hence, most of the properties of the AnimationPlayer will not be updated.

godot/scene/animation/animation_tree.cpp

Lines 526 to 537 in d2be503

	bool AnimationTree::_update_caches(AnimationPlayer *player) {
	setup_pass++;
	if (!player->has_node(player->get_root())) {
	ERR_PRINT("AnimationTree: AnimationPlayer root is invalid.");
	set_active(false);
	return false;
	}
	Node *parent = player->get_node(player->get_root());
	List<StringName> sname;
	player->get_animation_list(&sname);

As suggested by @Calinou in #34947, it would be good to mention this behavior in the AnimationTree's class documentation.

This closes #38956, closes #34947 and closes godotengine/godot-docs#3610

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.2.