Add examples to AnimationRootNode Description

doc/classes/AnimationRootNode.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		[AnimationRootNode] is a base class for [AnimationNode]s that hold a complete animation. A complete animation refers to the output of an [AnimationNodeOutput] in an [AnimationNodeBlendTree] or the output of another [AnimationRootNode]. Used for [member AnimationTree.tree_root] or in other [AnimationRootNode]s.
+		Examples of built-in root nodes include [AnimationNodeBlendTree] (allows blending nodes between each other using various modes), [AnimationNodeStateMachine] (allows to configure blending and transitions between nodes using a state machine pattern), [AnimationNodeBlendSpace2D] (allows linear blending between [b]three[/b] [AnimationNode]s), [AnimationNodeBlendSpace1D] (allows linear blending only between [b]two[/b] [AnimationNode]s).
 	</description>
 	<tutorials>
 		<link title="Using AnimationTree">$DOCS_URL/tutorials/animation/animation_tree.html</link>