message: Add grumpy admonishment to Canvas docs.
diff: @@ -29,9 +29,14 @@ where
  /// to as "render target" or "render to texture"). Set the canvas with the
  /// `ggez::graphics::set_canvas()` function, and then anything you
  /// draw will be drawn to the canvas instead of the screen.
  -///
  /// Resume drawing to the screen by calling `ggez::graphics::set_canvas(None)`.
  ///
  +/// **This is not an optimization tool.** You may be tempted to say "I'll draw a scene
  +/// to a `Canvas` and then just draw the single `Canvas` each frame." This is technically
  +/// possible but makes life much harder than it needs to be (especially since the current
  +/// implementation of `Canvas` has a number of [hard-to-squash bugs](https://github.com/ggez/ggez/issues?utf8=%E2%9C%93&q=is%3Aissue+canvas+).)
  +/// If you want to draw things maximally efficiently, use `SpriteBatch`.
  +///
  /// A `Canvas` allows creating render targets to be used instead of
  /// the screen. This allows graphics to be rendered to images off-screen
  /// in order to do things like saving to an image file or creating cool effects
  