Mention `SceneTree.create_timer()` in the Timer class documentation

This closes https://github.com/godotengine/godot-docs/issues/2349
(as the linked method already contains an example).

(cherry picked from commit 1261f47c35ec6963fccfc56e7e7f4bb2f288a691)

doc/classes/Timer.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		Counts down a specified interval and emits a signal on reaching 0. Can be set to repeat or "one-shot" mode.
+		[b]Note:[/b] To create an one-shot timer without instantiating a node, use [method SceneTree.create_timer].
 	</description>
 	<tutorials>
 	</tutorials>