So for kernel specific updates, this occurs when your system has downloaded and installed a new kernel package, there aren't any "updates" that are done to the kernel itself without a new package being installed, so running that command isn't going to get you anything useful.
The warning you get is for the system to boot into the kernel, in order to do this you need to reboot.
If you're wanting to understand what kernel update you're installing when you reboot you can look at the kernel packages on the system.