pgsql: Add documentation about the issues of casting MONEY to/from

Log Message:
-----------
Add documentation about the issues of casting MONEY to/from numeric
types.

Modified Files:
--------------
pgsql/doc/src/sgml:
datatype.sgml (r1.217 -> r1.218)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml?r1=1.217&r2=1.218)