Issue #3349395: Add description to TwitterUsernameDefaultFormatter

src/Plugin/Field/FieldFormatter/TwitterUsernameDefaultFormatter.php

@@ -11,6 +11,7 @@ use Drupal\Core\Field\FormatterBase;
  * @FieldFormatter(
  *   id = "twitter_username_default",
  *   label = @Translation("Default"),
+ *   description = @Translation('Displays twitter username as plain text.'),
  *   field_types = {
  *     "twitter_username",
  *   },