Add CompositionGraphicsDevice exception remark

Author: trympet

windows.ui.composition/compositiongraphicsdevice_createdrawingsurface_428570653.md

@@ -28,6 +28,7 @@ How the alpha channel should be handled.
 The created [CompositionDrawingSurface](compositiondrawingsurface.md).
 
 ## -remarks
+If the requested pixel format is unsupported by the graphics device, an exception will be thrown.
 
 ## -examples