Warn about Syslinux's reboot.c32 and poweroff.c32 requiring APM

configs/releng/syslinux/archiso_tail.cfg:
APM is old and some motherboard firmwares have removed support for it.
reboot.c32 and poweroff.c32 will not work on such firmwares.
Add a help text to "Reboot" and "Power Off" boot menu entries that warns about requiring APM.