drm/xe: Add kerneldoc description of multi-tile devices

v2:
 - Fix doubled word.  (Lucas)

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230601215244.678611-32-matthew.d.roper@intel.com


Signed-off-by: Matt Roper <matthew.d.roper@intel.com>