Update missing constant comment.

Refs #11113

src/Validation/ValidatorAwareTrait.php

@@ -25,11 +25,13 @@ use RuntimeException;
  * the implementing class wants to build and customize a variety
  * of validator instances.
  *
- * This trait expects that classes including it define two constants:
+ * This trait expects that classes including it define three constants:
  *
  * - `DEFAULT_VALIDATOR` - The default validator name.
  * - `VALIDATOR_PROVIDER_NAME ` - The provider name the including class is assigned
  *   in validators.
+ * - `BUILD_VALIDATOR_EVENT` - The name of the event to be triggred when validators
+ *   are built.
  *
  * If the including class also implements events the `Model.buildValidator` event
  * will be triggered when validators are created.