ACPI / hotplug / PCI: Proper kerneldoc comments for enumeration/removal

Add proper kerneldoc comments describing acpiphp_enumerate_slots()
and acpiphp_remove_slots().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>