Add php-intl dependency info for SluggableBehavior (#15997) [skip ci]

framework/behaviors/SluggableBehavior.php

@@ -17,6 +17,9 @@ use yii\validators\UniqueValidator;
 /**
  * SluggableBehavior automatically fills the specified attribute with a value that can be used a slug in a URL.
  *
+ * Note: This behavior relies on php-intl extension for transliteration. If it is not installed it
+ * falls back to replacements defined in [[\yii\helpers\Inflector::$transliteration]].
+ *
  * To use SluggableBehavior, insert the following code to your ActiveRecord class:
  *
  * ```php