message: Inform in Canvas doc about graphics::set_screen_coordinates
  If you render to a canvas that has a different aspect ratio than
  the screen, the results will look skewed.
diff: @@ -39,6 +39,12 @@ where
  /// by using shaders that render to an image.
  /// If you just want to draw multiple things efficiently, look at
  /// [`SpriteBatch`](spritebatch/struct.Spritebatch.html).
  +///
  +/// Note that if the canvas is not of the same size as the screen, and you want
  +/// to render using coordinates relative to the canvas' coordinate system, you
  +/// need to call [`graphics::set_screen_coordinates`](fn.set_screen_coordinates.html)
  +/// and pass in a rectangle with position (0, 0) and a size equal to that of the
  +/// canvas.
  pub type Canvas = CanvasGeneric<GlBackendSpec>;
  impl<S> CanvasGeneric<S>
  