diff -u b/core/lib/Drupal/Core/Session/AccountInterface.php b/core/lib/Drupal/Core/Session/AccountInterface.php
--- b/core/lib/Drupal/Core/Session/AccountInterface.php
+++ b/core/lib/Drupal/Core/Session/AccountInterface.php
@@ -106,22 +106,30 @@
   public function getPreferredAdminLangcode($fallback_to_default = TRUE);
 
   /**
-   * Returns the username of this account.
+   * Returns the unaltered login name of this account.
    *
    * @return
-   *   An unsanitized plain-text string with the username to display.
+   *   An unsanitized plain-text string with the name of this account that is
+   *   used to log in. Only display this name to admins and to the user who owns
+   *   this account, and only in the context of the name used to login. For
+   *   any other display purposes, use
+   *   \Drupal\Core\Session\AccountInterface::getDisplayName() instead.
    *
    * @deprecated in Drupal 8.0.0, will be removed before Drupal 9.0.0.
-   *   Use Drupal\Core\Session\AccountInterface::getAccountName() or
+   *   Use \Drupal\Core\Session\AccountInterface::getAccountName() or
    *   \Drupal\user\RoleInterface::getDisplayName() instead.
    */
   public function getUsername();
 
   /**
-   * Returns the unaltered name of this account.
+   * Returns the unaltered login name of this account.
    *
    * @return
-   *   An unsanitized plain-text string with the username to display.
+   *   An unsanitized plain-text string with the name of this account that is
+   *   used to log in. Only display this name to admins and to the user who owns
+   *   this account, and only in the context of the name used to login. For
+   *   any other display purposes, use
+   *   \Drupal\Core\Session\AccountInterface::getDisplayName() instead.
    */
   public function getAccountName();