Try to explain why one needs to set the namespaces feature to true

when validating documents
PR: 29187


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@276569 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/OptionalTasks/xmlvalidate.html

@@ -24,6 +24,21 @@ SAX1/2 parser if needed.</p>
   <li><tt>&lt;property&gt;</tt> elements, containing string properties
 </p>
 
+<p><b>Warning</b> : JAXP creates by default a non namespace aware parser.
+The <tt>"http://xml.org/sax/features/namespaces"</tt> feature is set
+by default to <tt>false</tt> by the JAXP implementation used by ant. To validate
+a document containing namespaces,
+set the namespaces feature to <tt>true</tt> explicitly by nesting the following element:
+<pre>
+  &lt;attribute name="http://xml.org/sax/features/namespaces" value="true"/&gt;
+</pre>
+If you are using for instance a <tt>xsi:noNamespaceSchemaLocation</tt> attribute in your XML files,
+you will need this namespace support feature.
+</p>
+<p>If you are using a parser not generated by JAXP, by using the <tt>classname</tt> attribute of xmlvalidate, this warning
+may not apply.</p>
+
+
 <h3>Parameters</h3>
 <table border="1" cellpadding="2" cellspacing="0">
   <tr>