core/modules/image/src/ImageEffectInterface.php | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/core/modules/image/src/ImageEffectInterface.php b/core/modules/image/src/ImageEffectInterface.php
index 790d7b2..7d2c4ca 100644
--- a/core/modules/image/src/ImageEffectInterface.php
+++ b/core/modules/image/src/ImageEffectInterface.php
@@ -37,9 +37,12 @@ public function applyEffect(ImageInterface $image);
   /**
    * Determines the dimensions of the styled image.
    *
-   * @param array $dimensions
-   *   Dimensions to be modified - an array with components width and height, in
-   *   pixels.
+   * @param array &$dimensions
+   *   Dimensions to be modified - an array with the following keys:
+   *   - width: the width in pixels, or NULL if unknown
+   *   - height: the height in pixels, or NULL if unknown
+   *   When either of the dimensions are NULL, the corresponding HTML attribute
+   *   will be omitted when an image style using this image effect is used.
    */
   public function transformDimensions(array &$dimensions);