pgsql: Improve COPY documentation to clarify that it doesn't copy data

Log Message:
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Improve COPY documentation to clarify that it doesn't copy data to or from
child tables. Per gripe from Jaime Casanova.

Modified Files:
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pgsql/doc/src/sgml/ref:
copy.sgml (r1.97 -> r1.98)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/copy.sgml?r1=1.97&r2=1.98)