date(1): Improve manpage around '-I'

• ISO 8601 defines two formats: basic and extended, clarify that we output the extended format only.
• Clarify that ISO 8601 is only aware of timezone /offsets/, not timezones, it has no relation to the Olson timezone database (TZ environment variable).

Reviewed by: otis (mentor), des
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52314