message: Update Canvas module docs a little
diff: -//! The `canvas` module enables 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.
  +//! I guess these docs will never appear since we re-export the canvas
  +//! module from graphics...
  use gfx::{Factory};
  use gfx::format::{ChannelTyped, Srgb, Srgba8, Swizzle};
  @@ -24,6 +23,11 @@ where
  image: Image,
  }
  +/// The `Canvas` type 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.
  +/// If you just want to draw multiple things efficiently, look at `SpriteBatch`.
  +///
  /// A canvas that can be rendered to instead of the screen (sometimes referred
  /// to as "render target" or "render to texture"). Set the canvas with the
  /// `ggez::graphics::set_canvas()` function, and then anything you
  