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.

Change-Id: I76c75dc951dfd6bcf86e8b0be3cfec518a3ecdee
Reviewed-on: https://go-review.googlesource.com/c/go/+/213977Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/time/time.go

@@ -1148,6 +1148,9 @@ func (t Time) Zone() (name string, offset int) {
 // Unix returns t as a Unix time, the number of seconds elapsed
 // since January 1, 1970 UTC. The result does not depend on the
 // location associated with t.
+// Unix-like operating systems often record time as a 32-bit
+// count of seconds, but since the method here returns a 64-bit
+// value it is valid for billions of years into the past or future.
 func (t Time) Unix() int64 {
 	return t.unixSec()
 }