Docs: mild blur is a box filter, add examples

lib/operation.js

@@ -245,8 +245,21 @@ function median (size) {
 
 /**
  * Blur the image.
- * When used without parameters, performs a fast, mild blur of the output image.
+ *
+ * When used without parameters, performs a fast 3x3 box blur (equivalent to a box linear filter).
+ *
  * When a `sigma` is provided, performs a slower, more accurate Gaussian blur.
+ *
+ * @example
+ * const boxBlurred = await sharp(input)
+ *   .blur()
+ *   .toBuffer();
+ *
+ * @example
+ * const gaussianBlurred = await sharp(input)
+ *   .blur(5)
+ *   .toBuffer();
+ *
  * @param {number} [sigma] a value between 0.3 and 1000 representing the sigma of the Gaussian mask, where `sigma = 1 + radius / 2`.
  * @returns {Sharp}
  * @throws {Error} Invalid parameters