Document the `BOOST_COMPUTE_CL_CALLBACK` and `BOOST_COMPUTE_MAX_ARITY` configuration macros. From the review: ``` <http://kylelutz.github.io/compute/BOOST_COMPUTE_CL_CALLBACK.html> is empty, as is <http://kylelutz.github.io/compute/BOOST_COMPUTE_MAX_ARITY.html> ``` ##