message: Direct user to DynamicImage for converting
  Attempts to address the documentation suggestion in
  A lot of the methods for modifying an image are found in the ImageBuffer
  so that is where I was looking when trying to convert an image to a
  different Pixel type.
diff: @@ -416,6 +416,8 @@ where
  /// [`RgbImage`]: type.RgbImage.html
  /// [`GrayImage`]: type.GrayImage.html
  ///
  +/// To convert between images of different Pixel types use [`DynamicImage`]: enum.DynamicImage.html
  +///
  /// ## Examples
  ///
  /// Create a simple canvas and paint a small cross.
  @@ -449,6 +451,15 @@ where
  ///
  /// image::imageops::overlay(&mut img, &on_top, 128, 128);
  /// ```
  +///
  +/// Convert an RgbaImage to a GrayImage.
  +///
  +/// ```no_run
  +/// use image::{open, DynamicImage};
  +///
  +/// let rgba = open("path/to/some.png").unwrap().into_rgba();
  +/// let gray = DynamicImage::ImageRgba8(rgba).into_luma();
  +/// ```
  #[derive(Debug, Hash, PartialEq, Eq)]
  pub struct ImageBuffer<P: Pixel, Container> {
  width: u32,
  