Update 21.Advanced_Topics_II.pdf

- Improve description of floating-point to integer undefined behavior
- fix `delete[]` -> `delete` in `unique_ptr` example
- Describe `make_shared` constructor vs. factory
- Fix `std::thread` example - parameters passing
- Use `lock_guard` in mutex example
- Specify that `std::atomic` could not be the fastest solution

Author: federico-busato