pgsql: Suggest Win32 users user E'' strings and double backslashes used

Log Message:
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Suggest Win32 users user E'' strings and double backslashes used for
patch separators in COPY.

Backpatch doubleing backslashes suggestion to 8.1.

Modified Files:
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pgsql/doc/src/sgml/ref:
copy.sgml (r1.73 -> r1.74)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/copy.sgml.diff?r1=1.73&r2=1.74)