doc: add description of isl_space_is_equal
doc: add description of isl_space_is_equal

A description of what isl_space_is_equal checks was missing.

Signed-off-by: Michael Kruse <8300014ab702124f340a1eb40c7d70d0a8f7b3f1@meinersbur.de>
Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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