doc: document isl_ast_expr_op_address_of
doc: document isl_ast_expr_op_address_of

This was missing from isl-0.13-132-gf5fcba3fc0 (Add an AST expression
representing the &-operator, Sat Aug 9 23:15:03 2014 -0700).

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

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