Try to clarify docs for ImageFormat

The image format is based on individual bytes and thus independent of
the system's endianness. This commit tries to make that more explicit.

This was originally suggested in issue linebender#224.

Signed-off-by: Uli Schlachter <psychon@znc.in>

piet/src/render_context.rs

@@ -22,13 +22,24 @@ pub enum InterpolationMode {
 #[derive(Clone, Copy, Debug, PartialEq)]
 #[non_exhaustive]
 pub enum ImageFormat {
-    /// 1 byte per pixel
+    /// 1 byte per pixel.
+    ///
+    /// For example, a white pixel has value 0xff.
     Grayscale,
     /// 3 bytes per pixel, in RGB order.
+    ///
+    /// For example, a red pixel consists of three bytes `[0xff, 0, 0]` independent of the system's
+    /// endianness.
     Rgb,
     /// 4 bytes per pixel, in RGBA order, with separate alpha.
+    ///
+    /// For example, a full-intensity red pixel with 50% transparency consists of four bytes
+    /// `[0xff, 0, 0, 0x80]` independent of the system's endianness.
     RgbaSeparate,
     /// 4 bytes per pixel, in RGBA order, with premultiplied alpha.
+    ///
+    /// For example, a full-intensity red pixel with 50% transparency consists of four bytes
+    /// `[0x80, 0, 0, 0x80]` independent of the system's endianness.
     RgbaPremul,
 }