Field type plugin description is assumed to be an array

Closes #3415412

core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldType/TestItemWithMultipleDescriptions.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\field_test\Plugin\Field\FieldType;
+
+/**
+ * Defines the 'test_field_with_multiple_descriptions' entity field item.
+ *
+ * @FieldType(
+ *   id = "test_field_with_multiple_descriptions",
+ *   label = @Translation("Test field (multiple descriptions"),
+ *   description = {
+ *     @Translation("This multiple line description needs to use an array"),
+ *     @Translation("This second line contains important information"),
+ *   },
+ *   category = "field_test_descriptions",
+ *   default_widget = "test_field_widget",
+ *   default_formatter = "field_test_default"
+ * )
+ */
+class TestItemWithMultipleDescriptions extends TestItem {
+}