Dokumentation for acpid can be found in 'man acpid'.

Be aware that the acpid is only used to hand over ACPI events
(caught from /proc/acpi/events) to other services
(e.g. hal, powersaved, iald, ...).

The events are handled by those services.
Most of them are processed by the powersave daemon.
For further documentation, please have a look at:
/usr/share/doc/packages/powersave/
or
http://powersave.sourceforge.net/