Resolve #3232488 "Deprecate transliteration plugin"

Closes #3232488

src/Plugin/migrate/process/Service.php

@@ -9,7 +9,7 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
 /**
  * Provides a plugin to use a callable from a service class.
  *
- * Example:
+ * Examples:
  *
  * @code
  * process:
@@ -20,6 +20,27 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
  *     source: filename
  * @endcode
  *
+ * @code
+ * source:
+ *   # plugin ...
+ *   constants:
+ *     langcode: en
+ *     slash: /
+ * process:
+ *   transliterated_value:
+ *     plugin: service
+ *     service: transliteration
+ *     method: transliterate
+ *     unpack_source: true
+ *     source:
+ *       - original_value
+ *       - constants/langcode
+ *       - constants/slash
+ * @endcode
+ *
+ * Since Drupal 9.2.0, it is possible to supply multiple arguments using
+ * unpack_source property. See: https://www.drupal.org/node/3205079
+ *
  * All options for the callback plugin can be used, except for 'callable',
  * which will be ignored.
  *