Fix description about SceneTreeTimer auto free

(cherry picked from commit 5a011f599907901788f0e5381e548b0062a940ed)

doc/classes/SceneTreeTimer.xml

@@ -12,7 +12,7 @@
 		    yield(get_tree().create_timer(1.0), "timeout")
 		    print("Timer ended.")
 		[/codeblock]
-		The timer will be automatically freed after its time elapses, so be aware that any reference you might have kept to it will become invalid.
+		The timer will be dereferenced after its time elapses. To preserve the timer, you can keep a reference to it. See [Reference].
 	</description>
 	<tutorials>
 	</tutorials>