Issue #1444650 by damiankloip: Make documentation of theme_image_style clearer

modules/image/image.module

@@ -1171,7 +1171,9 @@ function image_effect_apply($image, $effect) {
  *   - style_name: The name of the style to be used to alter the original image.
  *   - path: The path of the image file relative to the Drupal files directory.
  *     This function does not work with images outside the files directory nor
- *     with remotely hosted images.
+ *     with remotely hosted images. This should be in a format such as
+ *     'images/image.jpg', or using a stream wrapper such as
+ *     'public://images/image.jpg'.
  *   - width: The width of the source image (if known).
  *   - height: The height of the source image (if known).
  *   - alt: The alternative text for text-based browsers.