Document passing kernel command line arguments

[kousu]

I recently switched to using UKI, which as I understand normally copies the kernel command line from /etc/cmdline and embeds it into itself, but can read kernel line options from the efi boot entry; and if using EFISTUB using the extra options is mandatory to make a booting system.

The only documentation I could find online for how to actually make my system boot this way without a third party bootloader was in

• http://www.rodsbooks.com/efi-bootloaders/efistub.html
• https://wiki.archlinux.org/title/EFISTUB#efibootmgr

I think the manpage should explain this better!

The manpage nods at this feature with the docs for -@ and -u but doesn't mention in the argument summary that "extra command line arguments" exist, nor why you would care to use them.

[kousu]

If I need to write a better commit message or I messed up the manpage syntax or anything or I got anything factually wrong here let me know!