Clean up and comment add_native_events in papi_internal.c
Clean up and comment add_native_events in papi_internal.c

I'm chasing some weird perf_events behavior with
the papi_event_chooser.  The add_native_events
code is very hard to understand, working on
commenting it more.

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