Misleading C++ exception handling example

**Version of emscripten/emsdk:**
[Documentation](https://emscripten.org/docs/porting/Debugging.html#handling-c-exceptions-from-javascript) - all version from [here](https://github.com/emscripten-core/emscripten/pull/11073).

```cpp
std::string getExceptionMessage(intptr_t exceptionPtr) {
return std::string(reinterpret_cast(exceptionPtr)->what());
}
```

According to the documentation, this code snippet appears to be a handler function for getExceptionMessage. However, it is much simpler than the current implementation found [here](https://github.com/emscripten-core/emscripten/blob/3.1.42/system/lib/libcxxabi/src/cxa_exception_js_utils.cpp#L55).

The documentation also mentions that "this example code will only work for thrown statically allocated exceptions" but in reality, it only works if the exception type is exactly `std::exception`. It will not work (according to the standard C++) if the exception is a child of `std::exception` because `is_pointer_interconvertible_base_of_v` evaluates to false. Therefore, the reinterpret casting is not safe and results in undefined behavior.

Another issue is that this code can be exploited using the following structure:

```cpp
struct BaseException {
virtual ~BaseException() = default;
virtual const char* hack() const noexcept { return "HACKED"; }
};
struct MyEx : BaseException, std::exception {
virtual const char* what() const noexcept { return "This won't be returned"; }
};

throw MyEx();
```

---

[We](https://github.com/vizzuhq) are using Emscripten with the following switches: `-s DISABLE_EXCEPTION_CATCHING=1 -fno-rtti`. This means that we don't have a bound `__get_exception_message`, but we still have exceptions and we want to retrieve the reason using `.what()`. We thought that this code snippet would be sufficient for handling `.what()`, but it turns out it's not. Therefore, this part of the documentation is misleading.