Improve accessibility of close buttons in modals & alerts

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
document #13390
mark Android Firefox as officially supported
Wall of browser bugs: formatting tweaks
Merge pull request #13495 from twbs/android-firefox

Officially support Android Firefox
Merge pull request #13493 from twbs/close-btn-access

Improve accessibility of close buttons in modals & alerts
include RTL CSS CDN URL in Download docs
Merge branch 'master' of https://github.com/twbs/bootstrap into scrollspy-js-tests