Thread: pgsql: Reword exclusion constriants to mention that the operator can

Log Message:
-----------
Reword exclusion constriants to mention that the operator can return
false or null, per Tom.

Modified Files:
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    pgsql/doc/src/sgml:
        ddl.sgml (r1.92 -> r1.93)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ddl.sgml?r1=1.92&r2=1.93)