Fix #5003: LIBXML_XINCLUDE is not supported for non-pull parsers, should be documented (#5092)

Author: ndossche

reference/libxml/constants.xml

@@ -323,7 +323,7 @@
    </term>
    <listitem>
     <simpara>
-     Implement XInclude substitution
+     Perform XInclude substitution (only for pull parsers, i.e. <classname>XMLReader</classname>).
     </simpara>
    </listitem>
   </varlistentry>