pgsql: Re-add documentation for Win32 copy syntax for archive_command.

Log Message:
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Re-add documentation for Win32 copy syntax for archive_command.

Backpatch to 9.0.X.

Modified Files:
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pgsql/doc/src/sgml:
backup.sgml (r2.157 -> r2.158)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml?r1=2.157&r2=2.158)