isl_schedule_node_free: update documentation
isl_schedule_node_free: update documentation

Ever since 1eaa09c (isl_schedule: store pointer to leaf instead
of actual leaf object, Mon Feb 8 12:13:58 2016 +0100), any leaf tree
that a node may point to is an actual reference counted object
instead of pointer to a field inside the schedule, but the documentation
of this function was not updated accordingly.

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

Meinersbur/isl,tobig/isl,nicolasvasilache/isl,tobig/isl,PollyLabs/isl,PollyLabs/isl,nicolasvasilache/isl,PollyLabs/isl,nicolasvasilache/isl,UBERTC/isl,PollyLabs/isl,Meinersbur/isl,UBERTC/isl,UBERTC/isl,nicolasvasilache/isl,nicolasvasilache/isl,BenzoSM/isl,inducer/isl-mirror,BenzoSM/isl,BenzoSM/isl,Meinersbur/isl,BenzoSM/isl,BenzoSM/isl,inducer/isl-mirror,UBERTC/isl,inducer/isl-mirror,tobig/isl,PollyLabs/isl,inducer/isl-mirror,inducer/isl-mirror,tobig/isl,Meinersbur/isl