Issue #3395748 by KarimB, joachim: getSetting()'s documentation should specify...

Issue #3395748 by KarimB, joachim: getSetting()'s documentation should specify what happens when a setting doesn't exist

(cherry picked from commit d088a7b0)
(cherry picked from commit 7489ccac)

core/lib/Drupal/Core/Field/FieldStorageDefinitionInterface.php

@@ -74,7 +74,7 @@ public function getSettings();
    *   The setting name.
    *
    * @return mixed
-   *   The setting value.
+   *   The setting value or NULL if the setting name doesn't exist.
    */
   public function getSetting($setting_name);
 

core/lib/Drupal/Core/TypedData/DataDefinitionInterface.php

@@ -143,7 +143,7 @@ public function getSettings();
    *   The setting name.
    *
    * @return mixed
-   *   The setting value.
+   *   The setting value or NULL if the setting name doesn't exist.
    */
   public function getSetting($setting_name);