Mention default output node name for AnimationNodeBlendTree

(cherry picked from commit 6bf01e056743c39ebc9a678a504eca7158af7bb9)

doc/classes/AnimationNodeBlendTree.xml

@@ -4,7 +4,8 @@
 		[AnimationTree] node resource that contains many blend type nodes.
 	</brief_description>
 	<description>
-		This node may contain a sub-tree of any other blend type nodes, such as mix, blend2, blend3, one shot, etc. This is one of the most commonly used roots.
+		This node may contain a sub-tree of any other blend type nodes, such as [AnimationNodeTransition], [AnimationNodeBlend2], [AnimationNodeBlend3], [AnimationNodeOneShot], etc. This is one of the most commonly used roots.
+		An [AnimationNodeOutput] node named [code]output[/code] is created by default.
 	</description>
 	<tutorials>
 		<link>$DOCS_URL/tutorials/animation/animation_tree.html</link>