Adds textual caption to the close buttons. The "X" multiplication sign isn't very meaningful to screenreaders.

Credit: https://github.com/paypal/bootstrap-accessibility-plugin
Per:
    https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#alert
    https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#modal-dialog

possibly less confusing option, as per discussion, is to always show the
dropdown trigger button (both live and in that code example)

Assuming sr-only is used in the actual live code so that the menu can be
shown open already ... but having sr-only in the highlighted example
code itself is confusing/misleading.

Also, change the text for the button from "Dropdown" to "Dropdown
trigger" for clarity (and it then matches
http://getbootstrap.com/javascript/#dropdowns)

Second color stop value should be percentage instead of decimal value.

Addresses part of #13429.

Fixes #13395

Closes #13379.