Issue #3263877: DefaultEntityValue - document configuration properties and improve documentation

Closes #3263877

src/Plugin/migrate/process/DefaultEntityValue.php

@@ -8,9 +8,11 @@ use Drupal\migrate\Row;
 /**
  * Returns EntityLookup for a given default value if input is empty.
  *
- * @see \Drupal\migrate_plus\Plugin\migrate\process\EntityLookup
+ * Available configuration keys:
+ * - default_value: The default value that will be used as for entity lookup.
+ * For additional configuration keys, refer to the parent class.
  *
- * Example usage with full configuration:
+ * Example usage:
  * @code
  * process:
  *   uid:
@@ -26,6 +28,11 @@ use Drupal\migrate\Row;
  *       default_value: editorial
  * @endcode
  *
+ * In this example the user with "editorial" name will be assigned to 'uid'
+ * destination property if the original author was not found during lookup.
+ *
+ * @see \Drupal\migrate_plus\Plugin\migrate\process\EntityLookup
+ *
  * @MigrateProcessPlugin(
  *   id = "default_entity_value",
  *   handle_multiples = TRUE