Issue #2196821 by JiriK: Convert all calls & docs references to check_plain()...

Issue #2196821 by JiriK: Convert all calls & docs references to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in contact module.

core/modules/contact/lib/Drupal/contact/CategoryListController.php

@@ -6,6 +6,7 @@
 
 namespace Drupal\contact;
 
+use Drupal\Component\Utility\String;
 use Drupal\Core\Config\Entity\ConfigEntityListController;
 use Drupal\Core\Entity\EntityInterface;
 
@@ -35,7 +36,7 @@ public function buildRow(EntityInterface $entity) {
       $row['selected'] = t('No');
     }
     else {
-      $row['recipients'] = check_plain(implode(', ', $entity->recipients));
+      $row['recipients'] = String::checkPlain(implode(', ', $entity->recipients));
       $default_category = \Drupal::config('contact.settings')->get('default_category');
       $row['selected'] = ($default_category == $entity->id() ? t('Yes') : t('No'));
     }

core/modules/contact/lib/Drupal/contact/MessageFormController.php

@@ -7,6 +7,7 @@
 
 namespace Drupal\contact;
 
+use Drupal\Component\Utility\String;
 use Drupal\Core\Entity\ContentEntityFormController;
 use Drupal\Core\Language\Language;
 use Drupal\user\UserInterface;
@@ -61,12 +62,12 @@ public function form(array $form, array &$form_state) {
       $form['name']['#type'] = 'item';
       $form['name']['#value'] = $user->getUsername();
       $form['name']['#required'] = FALSE;
-      $form['name']['#markup'] = check_plain($user->getUsername());
+      $form['name']['#markup'] = String::checkPlain($user->getUsername());
 
       $form['mail']['#type'] = 'item';
       $form['mail']['#value'] = $user->getEmail();
       $form['mail']['#required'] = FALSE;
-      $form['mail']['#markup'] = check_plain($user->getEmail());
+      $form['mail']['#markup'] = String::checkPlain($user->getEmail());
     }
 
     // The user contact form has a preset recipient.

core/modules/contact/lib/Drupal/contact/MessageViewBuilder.php

@@ -9,6 +9,7 @@
 
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\EntityViewBuilder;
+use Drupal\Component\Utility\String;
 
 /**
  * Render controller for contact messages.
@@ -28,7 +29,7 @@ public function buildContent(array $entities, array $displays, $view_mode, $lang
         $entity->content['message'] = array(
           '#type' => 'item',
           '#title' => t('Message'),
-          '#markup' => check_plain($entity->getMessage()),
+          '#markup' => String::checkPlain($entity->getMessage()),
         );
       }
     }