Now that GRUB 2 is stable, kernel upgrade instructions should include the key commands to generate a new GRUB 2 config (as well as suggesting that name of the file copied follow the GRUB 2 expected conventions. Reproducible: Always
This is also needed in the Gentoo Handbook itself.
The kernel upgrade instructions are now available on the wiki at https://wiki.gentoo.org/wiki/Kernel/Upgrade so feel free to update the document there. The handbook does not need the grub(2)-mkconfig stuff just yet during the kernel configuration, as the bootloader configuration is handled a few chapters later.