It is unlikely that the value of UnixNano overflow in most
use cases. However, the max date of 2262 is also within the range
where it may be of concern to some users. Rather than have each
person recompute when this overflows to validate if its okay for
their use case, we just document it as within the years 1678 and
2262, for user's convenience.

Fixes #16977

Change-Id: I4988738c147f4a6d30f8b8735c3941b75113bb16
Reviewed-on: https://go-review.googlesource.com/28478Reviewed-by: Andrew Gerrand <adg@golang.org>