pgsql: Add an explicit comment about POSIX time zone names having the

Log Message:
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Add an explicit comment about POSIX time zone names having the reverse
sign convention from everyplace else in Postgres.  I don't suppose that
this will stop people from being confused, but at least we can say that
it's documented.

Modified Files:
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    pgsql/doc/src/sgml:
        datatype.sgml (r1.199 -> r1.200)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.199&r2=1.200)