doc: minor clarification of anchored subtrees
doc: minor clarification of anchored subtrees

Requested-by: Albert Cohen <73a0868e3038e0ec3c51dfdde902fd3aeb902a59@inria.fr>
Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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