feat：更新 GRAMMAR OF A DECLARATION MODIFIER

source/03_language_reference/06_Declarations.md

@@ -1885,6 +1885,9 @@ Swift 提供了五个级别的访问控制：`open`、`public`、`internal`、`f
 > 
 > 声明修饰符 → [访问级别修饰符](#access-level-modifier)
 > 
+> 声明修饰符 → [可变修饰符](#mutation-modifier)
+> 
+> 声明修饰符 → [actor 隔离修饰符](#actor-isolation-modifier)
 >  
 ####  declaration-modifiers {#declaration-modifiers}
 >