- Clarify how events are used and how the different event sysctl correlate

- Clarify how events are used and how the different event sysctl correlate
- Document how an event looks like that is received by devd(8) from acpi_ibm(4)
- Document the event codes as they occur on a T41p
- Add a note about sysctl.conf and an Xref to it
- Add an example section providing an example script and an entry for
  devd.conf. This is an adapted version of the one in acpi_fujitsu(4).
- s/Fn-F4/Fn+F4/ for consistency sake
- Add a button description where refering to a T41p, remove it otherwise,
  since it might vary from model to model
- Bump .Dd

Thanks for improvements to: brueffer, ru

Approved by:	brueffer