doc: clarify that isl_printer_get_str can only be used on a string printer
doc: clarify that isl_printer_get_str can only be used on a string printer

Requested-by: Uday R Bondhugula <3286ed62869d15a46a5727b96db473b4ccf30a94@gmail.com>
Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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