pgsql: CVS NULL Documentation Clearify documentation of CVS's output of

Log Message:
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CVS NULL Documentation

Clearify documentation of CVS's output of NULL values, per suggestion
from Magnus.

Backpatch to 8.4.X.

Tags:
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REL8_4_STABLE

Modified Files:
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pgsql/doc/src/sgml/ref:
copy.sgml (r1.85 -> r1.85.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/copy.sgml?r1=1.85&r2=1.85.2.1)