pgsql: Document that pg_dump -d/-D prevents invalid data from canceling

Log Message:
-----------
Document that pg_dump -d/-D prevents invalid data from canceling the
entire table load.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/doc/src/sgml/ref:
pg_dump.sgml (r1.81 -> r1.81.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_dump.sgml.diff?r1=1.81&r2=1.81.2.1)