Mention SubViewport.render_target_clear_mode property is intended for 2D usage.

doc/classes/SubViewport.xml

@@ -11,6 +11,7 @@
 	<members>
 		<member name="render_target_clear_mode" type="int" setter="set_clear_mode" getter="get_clear_mode" enum="SubViewport.ClearMode" default="0">
 			The clear mode when the sub-viewport is used as a render target.
+			[b]Note:[/b] This property is intended for 2D usage.
 		</member>
 		<member name="render_target_update_mode" type="int" setter="set_update_mode" getter="get_update_mode" enum="SubViewport.UpdateMode" default="2">
 			The update mode when the sub-viewport is used as a render target.