x86/reboot: Document the "reboot=pci" option

It is mentioned in the top level non-arch specific file but it was
overlooked here for x86.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210530162447.996461-2-paul.gortmaker@windriver.com