Issue #3447550 by brandonlira, sadamafridi, charlliequadros, joachim,...

Issue #3447550 by brandonlira, sadamafridi, charlliequadros, joachim, santhosh@21: defaultStorageSettings() and defaultFieldSettings() should document that they must not have setting names in common

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

@@ -249,6 +249,10 @@ public static function generateSampleValue(FieldDefinitionInterface $field_defin
   /**
    * Defines the storage-level settings for this plugin.
    *
+   * Setting names defined by this method must not duplicate the setting names
+   * returned by this plugin's implementation of defaultFieldSettings(), as
+   * both lists of settings are merged.
+   *
    * @return array
    *   A list of default settings, keyed by the setting name.
    */
@@ -257,6 +261,10 @@ public static function defaultStorageSettings();
   /**
    * Defines the field-level settings for this plugin.
    *
+   * Setting names defined by this method must not duplicate the setting names
+   * returned by this plugin's implementation of defaultStorageSettings(), as
+   * both lists of settings are merged.
+   *
    * @return array
    *   A list of default settings, keyed by the setting name.
    */