document mask values to be an octal

core/src/main/java/hudson/FilePath.java

@@ -1233,6 +1233,10 @@ public final class FilePath implements Serializable {
      * @param mask
      *      File permission mask. To simplify the permission copying,
      *      if the parameter is -1, this method becomes no-op.
+     *      <p>
+     *      please note mask is expected to be an octal if you use <a href="http://en.wikipedia.org/wiki/Chmod">chmod command line values</a>,
+     *      so preceded by a '0' in java notation, ie <code>chmod(0644)</code>
+     *
      * @since 1.303
      * @see #mode()
      */