http://www.w3.org/TR/wai-aria/roles#separator

Closes #15915 by merging it.

plus one example that still used the old <div role="navigation"> and a
fix-up of the callout as per
https://github.com/twbs/bootstrap/issues/15109

[skip sauce]

adding explicit aria-expanded="false" initial state (so they're
correctly announced by AT as being collapsed), and - in the case of
old-school use of <a> - role="button"

Refs #13554.
Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#tab-panel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

[skip sauce]

Follow-up to e9374c06
Per https://github.com/paypal/bootstrap-accessibility-plugin/commit/f203dea4b6d17264d7b776f02feaeeaa386cc9b2
Credit: https://github.com/paypal/bootstrap-accessibility-plugin & @rohk

[skip sauce]

1. Add aria-haspopup="true" and aria-expanded="false" to dropdown docs
2. Toggle aria-expanded between "true" and "false" when opening and closing dropdown

Satisfy item 1 from https://github.com/paypal/bootstrap-accessibility-plugin#dropdown. See #13553.