Update javadoc according to the code

Closes #157

src/main/java/org/codehaus/plexus/archiver/zip/AddedDirs.java

@@ -88,7 +88,7 @@ public void clear()
      *
      * @param vPath The path to add.
      *
-     * @return true if the path was not present, false if it already existed.
+     * @return true if the path was already present, false if it has been added.
      */
     public boolean update( String vPath )
     {