xive: Documentation updates
xive: Documentation updates

Correct the documentation in a couple of places to match the
actual behaviour and improve bits and pieces of it

Signed-off-by: Benjamin Herrenschmidt <a7089bb6e7e92505d88aaff006cbdd60cc9120b6@kernel.crashing.org>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

shenki/skiboot,open-power/skiboot,open-power/skiboot,qemu/skiboot,stewart-ibm/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,open-power/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot,legoater/skiboot,legoater/skiboot,stewart-ibm/skiboot,shenki/skiboot,stewart-ibm/skiboot,shenki/skiboot,qemu/skiboot,qemu/skiboot,qemu/skiboot