Document PCI hotplug resource reservation parameters hpiosize and
hpmemsize.  These parameters override default hotplug I/O size (256 bytes)
and default mem size (2M).
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>