Refactoring authorization docs (#341)

Co-authored-by: Sergei Predvoditelev <sergei@predvoditelev.ru>

Author: klsoft-web

src/guide/security/authorization.md

@@ -396,9 +396,10 @@ public function signup()
         $user->save(false);
 
         // the following three lines were added:
-        $auth = \Yii::$app->authManager;
-        $authorRole = $auth->getRole('author');
-        $auth->assign($authorRole, $user->getId());
+        $authorRole = $this->manager->getRole('author');
+        if ($authorRole !== null) {
+            $this->manager->assign($authorRole->getName(), $user->getId());
+        }
 
         return $user;
     }
@@ -408,7 +409,7 @@ public function signup()
 ```
 
 For applications that require complex access control with dynamically updated authorization data
-(such as an admin panel), you many need to develop special user interfaces using APIs offered by `authManager`.
+(such as an admin panel), you many need to develop special user interfaces using APIs offered by `Yiisoft\Rbac\Manager`.
 
 
 ### Using rules <span id="using-rules"></span>