more details in upgrade notes

framework/UPGRADE.md

@@ -108,13 +108,13 @@ Upgrade from Yii 2.0.12
   require `"yiisoft/yii2": "~2.0.13"` in composer.json and change affected classes to extend from `yii\base\BaseObject`
   instead. It is not possible to allow Yii versions `<2.0.13` and be compatible with PHP 7.2 or higher.
 
-* A new method `public static function instance($refresh = false);` has been added to the `yii\db\ActiveRecordInterface`.
-  This change may affect your application in the following ways:
+* A new method `public static function instance($refresh = false);` has been added to the `yii\db\ActiveRecordInterface` via a new
+  `yii\base\StaticInstanceInterface`. This change may affect your application in the following ways:
 
   - If you have an `instance()` method defined in an `ActiveRecord` or `Model` class, you need to check whether the behavior is
     compatible with the method added by Yii.
   - Otherwise this method is implemented in the `yii\base\Model`, so the change only affects your code if you implement `ActiveRecordInterface`
-    in a class that does not extend `Model`.
+    in a class that does not extend `Model`. You may use `yii\base\StaticInstanceTrait` to implement it.
 
 Upgrade from Yii 2.0.11
 -----------------------

framework/base/StaticInstanceInterface.php

@@ -13,8 +13,11 @@ namespace yii\base;
  * For example: adjustments made by DI or behaviors reveal only at object level, but might be needed
  * at class (static) level as well.
  *
+ * To implement the [[instance()]] method you may use [[StaticInstanceTrait]].
+ *
  * @author Paul Klimov <klimov.paul@gmail.com>
  * @since 2.0.13
+ * @see StaticInstanceTrait
  */
 interface StaticInstanceInterface
 {