Add documentation for scene.background support for WebGLRenderTargetCube

docs/api/en/scenes/Scene.html

@@ -41,7 +41,7 @@
 
 		<h3>[property:Object background]</h3>
 		<p>
-		If not null, sets the background used when rendering the scene, and is always rendered first. Can be set to a [page:Color] which sets the clear color, a [page:Texture] covering the canvas, or a [page:CubeTexture]. Default is null.
+		If not null, sets the background used when rendering the scene, and is always rendered first. Can be set to a [page:Color] which sets the clear color, a [page:Texture] covering the canvas, or a cubemap as a [page:CubeTexture] or [page:WebGLRenderTargetCube]. Default is null.
 		</p>
 
 		<h2>Methods</h2>