image.api.php cleanup

Found a couple of issues with the image.module hooks_ documentation.

1. Documentation for hook_image_style_flush() makes reference to a non-existent UI for flushing a style.

2. Documentation for hook_image_style_alter() argument should be passed by reference.

3. Example function for hook_image_style_alter() is missing code. The example needs to include an 'effect callback' key in the array. It is required, and without it Drupal throws errors. The php docs for the function should probably clarify this as well.