consider moving the OpenCL C prefetch function to a different section

[bashbaug]

The OpenCL C spec currenlty has a single section "6.15.11. Async Copies from Global to Local Memory, Local to Global Memory, and Prefetch". It makes sense to put the async copies (and wait group events) functions in the same section since they're related and work together but prefetch seems like an odd fit for this section. We should either find another place for it or add it to its own section.