Expand caveat for disabled fieldsets and links
Redundant role="form" on <form>s
Redundant role="navigation" on <nav>s

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
Merge pull request #15125 from patrickhlauke/docs-clean-up

Docs clean up - unnecessary role attributes, callout clarifications