Specify the behaviour of get_tree when the node is not in the scene tree

(cherry picked from commit ee9c8cb2967d6bbe68550cdaa2615f6f053c94cf)

doc/classes/Node.xml

@@ -445,7 +445,7 @@
 		<method name="get_tree" qualifiers="const">
 			<return type="SceneTree" />
 			<description>
-				Returns the [SceneTree] that contains this node.
+				Returns the [SceneTree] that contains this node. Returns [code]null[/code] and prints an error if this node is not inside the scene tree. See also [method is_inside_tree].
 			</description>
 		</method>
 		<method name="get_viewport" qualifiers="const">