Improve docs relating to single-channel raw pixel output

lib/output.js

@@ -517,7 +517,9 @@ function heif (options) {
 }
 
 /**
- * Force output to be raw, uncompressed uint8 pixel data.
+ * Force output to be raw, uncompressed, 8-bit unsigned integer (unit8) pixel data.
+ * Pixel ordering is left-to-right, top-to-bottom, without padding.
+ * Channel ordering will be RGB or RGBA for non-greyscale colourspaces.
  *
  * @example
  * // Extract raw RGB pixel data from JPEG input
@@ -525,6 +527,15 @@ function heif (options) {
  *   .raw()
  *   .toBuffer({ resolveWithObject: true });
  *
+ * @example
+ * // Extract alpha channel as raw pixel data from PNG input
+ * const data = await sharp('input.png')
+ *   .ensureAlpha()
+ *   .extractChannel(3)
+ *   .colourspace('b-w')
+ *   .raw()
+ *   .toBuffer();
+ *
  * @returns {Sharp}
  */
 function raw () {