book: discriminative modification to the nature of override and overl…

…oad (#265)

book/en-us/02-usability.md

@@ -971,7 +971,7 @@ C++11 introduces the two keywords `override` and `final` to prevent this from ha
 
 ### override
 
-When overriding a virtual function, introducing the `override` keyword will explicitly tell the compiler to overload, and the compiler will check if the base function has such a virtual function, otherwise it will not compile:
+When overriding a virtual function, introducing the `override` keyword will explicitly tell the compiler to overload, and the compiler will check if the base function has such a virtual function with consistent function signature, otherwise it will not compile:
 
 ```cpp
 struct Base {

book/zh-cn/02-usability.md

@@ -880,7 +880,7 @@ C++11 引入了 `override` 和 `final` 这两个关键字来防止上述情形
 
 #### override
 
-当重载虚函数时，引入 `override` 关键字将显式的告知编译器进行重载，编译器将检查基函数是否存在这样的虚函数，否则将无法通过编译：
+当重载虚函数时，引入 `override` 关键字将显式的告知编译器进行重载，编译器将检查基函数是否存在这样的其函数签名一致的虚函数，否则将无法通过编译：
 
 ```cpp
 struct Base {