GH-1544: Update LeshanServerBuilder.setUpdateRegistrationOnSend javadoc.

leshan-server-core/src/main/java/org/eclipse/leshan/server/LeshanServerBuilder.java

@@ -289,6 +289,8 @@ public LeshanServerBuilder setUpdateRegistrationOnNotification(boolean updateReg
  * <p>
  * With this option you can allow Leshan to update Registration on Send Operation. This is clearly OUT OF
  * SPECIFICATION and so this is not recommended and should be used only if there is no other way.
+ * <p>
+ * This only works when using protocol with authentication, e.g. this can not work with {@code coap://}.
  *
  * @see <a href=
  * "https://github.com/eclipse/leshan/wiki/LWM2M-Devices-with-Dynamic-IP#is-the-update-request-mandatory--should-i-update-registration-on-notification-">Dynamic