documentation update for doc/src/sgml/func.sgml

Hi,

attached is a small patch which updates doc/src/sgml/func.sgml. The
change explains that functions like round() and others might behave
different depending on your operating system (because of rint(3)) and
that this is according to an IEEE standard. It also points out that #.5
is not always rounded up, as expected from a mathematical point of view.


Regards,

--
                Andreas 'ads' Scherbaum
German PostgreSQL User Group
European PostgreSQL User Group - Board of Directors
Volunteer Regional Contact, Germany - PostgreSQL Project