freebsd-update: Mention upgrading packages, not just ports

These days most end users are likely using pre-built packages, not
locally-built ports. Thus be sure to mention this as an important case,
and put it ahead of ports.

Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42968

(cherry picked from commit ced74610217ff0be071e4614c64a4ff66c40bc6b)