date: attempt to more accurately describe year limitations with -v

The previous description was both incorrect and incomplete in its
description -- the 2038 limit doesn't apply on !i386 platforms, and
it didn't note that values above 100 are accepted and interpreted
differently. Further, it didn't note that absolute years are accepted.

Reviewed by: pauamma_gundo.com (manpages)
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35360