time: add a sentence about distant times to time.Time.Unix
Since Durations only span 290 years, they are not good for
manipulating very remote times. I bounced off this problem recently
while doing some astronomical calculations and it took me a while to
realize I could get a 64-bit seconds value from time.Time.Unix and
subtract two of them to evaluate the interval.
I thought it worth adding a sentence to make this clear. It didn't
occur to me for quite a while that "Unix time" spans a huge range in
the Go library.