Document missing configuration macros

[kylelutz]

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>