book: fix contradictory description of foo(NULL) overload resolution

book/en-us/02-usability.md

@@ -40,7 +40,7 @@ void foo(char*);
 void foo(int);
 ```
 
-Then the `foo(NULL);` statement will call `foo(int)`, which will cause the code to be counterintuitive.
+Then the behavior of the `foo(NULL);` statement depends on how `NULL` is implemented: when `NULL` is defined as `0` (for example, on MSVC), it calls `foo(int)`, which is counterintuitive; when `NULL` is defined as the GCC/Clang builtin `__null`, the call `foo(NULL)` becomes ambiguous between the `char*` and `int` overloads and fails to compile. In either case, `NULL` does not behave the way a proper null pointer should during overload resolution.
 
 To solve this problem, C++11 introduced the `nullptr` keyword, which is specifically used to distinguish null pointers, `0`. The type of `nullptr` is `nullptr_t`, which can be implicitly converted to any pointer or member pointer type, and can be compared equally or unequally with them.
 

book/zh-cn/02-usability.md

@@ -29,7 +29,7 @@ void foo(char*);
 void foo(int);
 ```
 
-那么 `foo(NULL);` 这个语句将会去调用 `foo(int)`，从而导致代码违反直觉。
+那么 `foo(NULL);` 这个语句的行为将取决于 `NULL` 的具体实现：当 `NULL` 被定义为 `0` 时（例如 MSVC），它会去调用 `foo(int)`，从而导致代码违反直觉；而当 `NULL` 被定义为 GCC/Clang 的内建常量 `__null` 时，`foo(NULL)` 则会因为 `char*` 与 `int` 两个重载都能匹配而产生二义性，导致编译失败。无论哪种情况，`NULL` 在重载决议中的表现都不符合空指针应有的语义。
 
 为了解决这个问题，C++11 引入了 `nullptr` 关键字，专门用来区分空指针、`0`。而 `nullptr` 的类型为 `nullptr_t`，能够隐式的转换为任何指针或成员指针的类型，也能和他们进行相等或者不等的比较。