Document POSIX time zone sign oddity more visibly

Appendix B already mentioned that the sign of the time zone offset
in POSIX time zones is the opposite of the normal time zone offset
sign.  However, this was somewhat hidden, so add a note to the
data type section and an example and a reference to the documentation
of AT TIME ZONE.
Reported by Chris BSomething.

Author: Aleksander Alekseev
Author: Laurenz Albe
Discussion: https://postgr.es/m/CADrHaBFMWsMxGoPBPqY570HSDQZeJ3BtJHhSbUg5LFdErZX6ug%40mail.gmail.com

Author: laurenz