Improved note on process.hrtime()

I just had trouble understanding [process.hrtime()](http://nodejs.org/api/process.html#process_process_hrtime). I thought `process.hrtime()` returns the time in two units, seconds and nanoseconds. It took me some time to figure out that `time[0] * 1000000000 + time[1]` is actually the overall result in nanoseconds.

You may correct me if my English is bad, but please add a note on this. Don't want others to do the same mistake :smile: