Clarify that create_timer does not require cleanup.

This is how I would expect it to work, but the docs didn't clarify, so I
had to check the source just to make sure I wasn't responsible for
freeing the timer:

d39f6386ce/scene/main/scene_tree.cpp (L473)
(cherry picked from commit 6e7e4f82ec)

doc/classes/SceneTree.xml

@@ -77,6 +77,7 @@
 				    yield(get_tree().create_timer(1.0), "timeout")
 				    print("end")
 				[/codeblock]
+				The timer will be automatically freed after its time elapses.
 			</description>
 		</method>
 		<method name="get_frame" qualifiers="const">