time: clarify that monotonic clock is not returned by t.Unix
Prompted by CL 403996.
Change-Id: I8b480a9d361863d4770c7c9a4437431424f26994
Reviewed-on: https://go-review.googlesource.com/c/go/+/404695
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Heschi Kreinick <heschi@google.com>