add role="alert" to Alert examples, to improve accessibility

Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/d8fcbcd3fe86f0cb47fda0ffe269b2c7a2131f07/README.md#alert
rework docs copy: inputs-without-labels are likely the more common case
Gruntfile.js: fix path after d08bbdc6ec50baca9e34d2aa6e8bcfc8d6135217.
Merge pull request #13024 from twbs/role-alert

add role="alert" to Alert examples, to improve accessibility
Update dependencies.