Clarify usage of CompositorEffect as an abstract base class in the documentation

doc/classes/CompositorEffect.xml

@@ -4,7 +4,7 @@
 		This resource allows for creating a custom rendering effect.
 	</brief_description>
 	<description>
-		This resource defines a custom rendering effect that can be applied to [Viewport]s through the viewports' [Environment]. You can implement a callback that is called during rendering at a given stage of the rendering pipeline and allows you to insert additional passes. Note that this callback happens on the rendering thread.
+		This resource defines a custom rendering effect that can be applied to [Viewport]s through the viewports' [Environment]. You can implement a callback that is called during rendering at a given stage of the rendering pipeline and allows you to insert additional passes. Note that this callback happens on the rendering thread. CompositorEffect is an abstract base class and must be extended to implement specific rendering logic.
 	</description>
 	<tutorials>
 	</tutorials>