Issue #3185528: "Callback" process plugin has removed method in the plugin description

core/modules/migrate/src/Plugin/migrate/process/Callback.php

@@ -22,7 +22,7 @@
  * process:
  *   destination_field:
  *     plugin: callback
- *     callable: strtolower
+ *     callable: mb_strtolower
  *     source: source_field
  * @endcode
  *
@@ -34,7 +34,7 @@
  *     plugin: callback
  *     callable:
  *       - '\Drupal\Component\Utility\Unicode'
- *       - strtolower
+ *       - ucfirst
  *     source: source_field
  * @endcode
  *