Clarify that CanvasLayers are restricted to one Viewport

doc/classes/CanvasLayer.xml

@@ -7,6 +7,7 @@
 		[CanvasItem]-derived nodes that are direct or indirect children of a [CanvasLayer] will be drawn in that layer. The layer is a numeric index that defines the draw order. The default 2D scene renders with index [code]0[/code], so a [CanvasLayer] with index [code]-1[/code] will be drawn below, and a [CanvasLayer] with index [code]1[/code] will be drawn above. This order will hold regardless of the [member CanvasItem.z_index] of the nodes within each layer.
 		[CanvasLayer]s can be hidden and they can also optionally follow the viewport. This makes them useful for HUDs like health bar overlays (on layers [code]1[/code] and higher) or backgrounds (on layers [code]-1[/code] and lower).
 		[b]Note:[/b] Embedded [Window]s are placed on layer [code]1024[/code]. [CanvasItem]s on layers [code]1025[/code] and higher appear in front of embedded windows.
+		[b]Note:[/b] Each [CanvasLayer] is drawn on one specific [Viewport] and cannot be shared between multiple [Viewport]s, see [member custom_viewport]. When using multiple [Viewport]s, for example in a split-screen game, you need create an individual [CanvasLayer] for each [Viewport] you want it to be drawn on.
 	</description>
 	<tutorials>
 		<link title="Viewport and canvas transforms">$DOCS_URL/tutorials/2d/2d_transforms.html</link>

doc/classes/ParallaxBackground.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		A ParallaxBackground uses one or more [ParallaxLayer] child nodes to create a parallax effect. Each [ParallaxLayer] can move at a different speed using [member ParallaxLayer.motion_offset]. This creates an illusion of depth in a 2D game. If not used with a [Camera2D], you must manually calculate the [member scroll_offset].
+		[b]Note:[/b] Each [ParallaxBackground] is drawn on one specific [Viewport] and cannot be shared between multiple [Viewport]s, see [member CanvasLayer.custom_viewport]. When using multiple [Viewport]s, for example in a split-screen game, you need create an individual [ParallaxBackground] for each [Viewport] you want it to be drawn on.
 	</description>
 	<tutorials>
 	</tutorials>