Disclaim support for default namespace in XMLTABLE

Pavel Stehule's original patch had support for default namespace, but I
ripped it out before commit -- hence the docs were correct when written,
and I broke them by omission :-(.  Remove the offending phrase.

Author: Daniel Gustafsson
Discussion: https://postgr.es/m/1550C5E5-FC70-4493-A226-AA137D831E8D@yesql.se

doc/src/sgml/func.sgml

@@ -10903,7 +10903,7 @@ SELECT xmltable.*
     <para>
      The following example illustrates how
      the <literal>XMLNAMESPACES</literal> clause can be used to specify
-     the default namespace, and a list of additional namespaces
+     a list of namespaces
      used in the XML document as well as in the XPath expressions:
 
   <screen><![CDATA[