Documentation needs to states class needed in paper-listbox

For Polymer 1.* and `paper-dropdown-menu` **before** 2.0

Documentation needs to state that `paper-listbox` has to have `class="dropdown-content"` for `paper-dropdown-menu` to drop down. Without that class, it will not drop down. 


As for ver 2.*, it doesn't work at all but that is outside of the context for this documentation PR. 