Add Tree iteration instructions thorugh TreeItem

doc/classes/Tree.xml

@@ -16,6 +16,7 @@
 		    var subchild1 = tree.create_item(child1)
 		    subchild1.set_text(0, "Subchild1")
 		[/codeblock]
+		To iterate over all the [TreeItem] objects in a [Tree] object, use [method TreeItem.get_next] and [method TreeItem.get_children] after getting the root through [get_root].
 	</description>
 	<tutorials>
 	</tutorials>