spapr: update spapr hotplug documentation

This updates the existing documentation to reflect recent updates to
the hotplug event structure, which are in draft form but slated
for inclusion in PAPR/LoPAPR.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>