cpp_generator::impl_printer::print_exceptional_execution_check: update doc
cpp_generator::impl_printer::print_exceptional_execution_check: update doc

This was missing from isl-0.23-131-gd22e0fdc1f (move
print_exceptional_execution_check to cpp_generator::impl_printer,
Sun Jan 10 12:08:29 2021 +0100).

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl