Add documentation about nested YSort nodes.

(cherry picked from commit ed3586afa1b09ef4a7d25deaf053cdad947a1cae)

doc/classes/YSort.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		Sort all child nodes based on their Y positions. The child node must inherit from [CanvasItem] for it to be sorted. Nodes that have a higher Y position will be drawn later, so they will appear on top of nodes that have a lower Y position.
+		Nesting of YSort nodes is possible. Children YSort nodes will be sorted in the same space as the parent YSort, allowing to better organize a scene or divide it in multiple ones, yet keep the unique sorting.
 	</description>
 	<tutorials>
 	</tutorials>