levelzero.h: we always return the root/parent device, not a subdevice

Refs #698

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
(cherry picked from commit cb3a260)

include/hwloc/levelzero.h

@@ -106,6 +106,10 @@ hwloc_levelzero_get_device_cpuset(hwloc_topology_t topology __hwloc_attribute_un
  * topology. If not, the locality of the object may still be found using
  * hwloc_levelzero_get_device_cpuset().
  *
+ * \note If the input L0 device is actually a subdevice, then its parent
+ * (root device) is actually translated, i.e. the main hwloc OS device
+ * is returned instead of one of its children.
+ *
  * \note The corresponding hwloc PCI device may be found by looking
  * at the result parent pointer (unless PCI devices are filtered out).
  */