pgsql: Remove munging of xml and xpath params to xpath().

Log Message:
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Remove munging of xml and xpath params to xpath(). The XML must now be a well formed XML document.

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.472 -> r1.473)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.472&r2=1.473)
pgsql/src/backend/utils/adt:
xml.c (r1.83 -> r1.84)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.83&r2=1.84)