Documentation needs to states class needed in paper-listbox

[dman777]

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.