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

Closes #15915 by merging it.

Fixes #15882 — Use an ID as the example for Scrollspy so that it is more clear that this is for use with a single nav.

Refs #11410

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"

Per https://github.com/paypal/bootstrap-accessibility-plugin#tab-panel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin