Docs: add stats usage note and example (#2803) (#2804)

Clarify that stats are derived from the original input image and that operations need to be buffered in order to obtain stats from the resulting image.

lib/input.js

@@ -358,6 +358,9 @@ function metadata (callback) {
  * - `sharpness`: Estimation of greyscale sharpness based on the standard deviation of a Laplacian convolution, discarding alpha channel if any (experimental)
  * - `dominant`: Object containing most dominant sRGB colour based on a 4096-bin 3D histogram (experimental)
  *
+ * **Note**: Statistics are derived from the original input image. Any operations performed on the image must first be
+ * written to a buffer in order to run `stats` on the result (see third example).
+ *
  * @example
  * const image = sharp(inputJpg);
  * image
@@ -370,6 +373,13 @@ function metadata (callback) {
  * const { entropy, sharpness, dominant } = await sharp(input).stats();
  * const { r, g, b } = dominant;
  *
+ * @example
+ * const image = sharp(input);
+ * // store intermediate result
+ * const part = await image.extract(region).toBuffer();
+ * // create new instance to obtain statistics of extracted region
+ * const stats = await sharp(part).stats();
+ *
  * @param {Function} [callback] - called with the arguments `(err, stats)`
  * @returns {Promise<Object>}
  */