gpu_array_tile: document that such an object represents a tile of an outer array
gpu_array_tile: document that such an object represents a tile of an outer array

gpu_array_tile objects are only used to represent tiles of
a gpu_array_info.  Since a gpu_array_info represents an outer tile,
a gpu_array_tile necessarily represents a tile of an outer array.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

Meinersbur/ppcg,Meinersbur/ppcg,nicolasvasilache/ppcg,nicolasvasilache/ppcg