javadoc added

platform/util/src/com/intellij/openapi/util/WriteExternalException.java

@@ -15,6 +15,10 @@
  */
 package com.intellij.openapi.util;
 
+/**
+ * Throw this exception from {@link JDOMExternalizable#writeExternal(org.jdom.Element)} method if you don't want to store any settings.
+ * If you simply return from the method empty '<component name=... />' tag will be written leading to unneeded modification of configuration files.
+ */
 public class WriteExternalException extends Exception {
   public WriteExternalException() {
     super();