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.

configs/releng/syslinux/archiso_tail.cfg

@@ -19,9 +19,17 @@ COM32 boot/syslinux/hdt.c32
 APPEND modules_alias=boot/syslinux/hdt/modalias.gz pciids=boot/syslinux/hdt/pciids.gz
 
 LABEL reboot
+TEXT HELP
+Reboot computer.
+The computer's firmware must support APM.
+ENDTEXT
 MENU LABEL Reboot
 COM32 boot/syslinux/reboot.c32
 
 LABEL poweroff
+TEXT HELP
+Power off computer.
+The computer's firmware must support APM.
+ENDTEXT
 MENU LABEL Power Off
 COM32 boot/syslinux/poweroff.c32