Add better documentation for the atomic_* functions

[kylelutz]

Add better documentation for the atomic_* functions. Indicate that they forward to their corresponding OpenCL functions and link that documentation.

From the review:

<http://kylelutz.github.io/compute/boost/compute/field.html> My
reading of this is that when you get the type wrong you are guaranteed
some kind of meaningful error report, and not just undefined behaviour;
is that right?  Perhaps you should clarify.