Document sleep button, lid status and lidaction/pwraction support

Document sleep button, lid status and lidaction/pwraction support

landry added the sensor back in 2013 and suspend via sleep button also works
(at least on ThinkPads).

`machdep.{lid,pwr}action' are super useful but only mentioned in
/etc/examples/wsconsctl.conf providing poor discovery.

acpibtn(4) is the most prominent driver supporting them, so document their
behaviour here as a reference point (`man -k .=lidaction' now shows it).

suspend/hibernate wording is taken from apm(8).
sysctl value list style is taken from sysctl(2)'s KERN_POOL_DEBUG.

Feedback jmc