Add doxygen comments for PAPI_add_events() +++ Updated PAPI_accum()
Add doxygen comments for PAPI_add_events() +++ Updated PAPI_accum()

pyrovski/papi,pyrovski/papi,pyrovski/papi,pyrovski/papi,arm-hpc/papi,arm-hpc/papi,arm-hpc/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi,arm-hpc/papi,arm-hpc/papi