pgsql: Document more clearly on XML namespaces inside xpath function

Log Message:
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Document more clearly on XML namespaces inside xpath function

Nikolay Samokhvalov

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.520 -> r1.521)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.520&r2=1.521)