doc: break up paragraph describing isl_access_info_compute_flow arguments
doc: break up paragraph describing isl_access_info_compute_flow arguments

This will allow some information to be added in an upcoming commit
without making the paragraph overly long.

This commit includes some minor rephrasing to avoid overfull lines.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

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