added a note about the data structure evolution rule.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@1184 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/XmlFile.java

@@ -2,6 +2,7 @@ package hudson;
 
 import com.thoughtworks.xstream.XStream;
 import com.thoughtworks.xstream.converters.ConversionException;
+import com.thoughtworks.xstream.converters.Converter;
 import com.thoughtworks.xstream.io.StreamException;
 import com.thoughtworks.xstream.io.xml.XppReader;
 import hudson.util.AtomicFileWriter;
@@ -57,7 +58,11 @@ import java.io.Reader;
  * In some limited cases (specifically when the class is the root object
  * to be read from XML, such as {@link Descriptor}), it is posible
  * to completely and drastically change the data format. See
- * {@link Descriptor#load()} for more about this technique.  
+ * {@link Descriptor#load()} for more about this technique.
+ *
+ * <p>
+ * There's a few other possibilities, such as implementing a custom
+ * {@link Converter} for XStream, or {@link XStream#alias(String, Class) registering an alias}.  
  *
  * @author Kohsuke Kawaguchi
  */