time: document that Parse truncates to nanosecond precision

For #48685
Fixes #50806

Change-Id: Ie8be40e5794c0998538890a651ef8ec92cb72d3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/381155


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
Reviewed-by: Michael Knyszek <mknyszek@google.com>