Use autocomplete="off" in buttons plugin docs examples

Refs #13875.
Have HTML5 validator ignore `autocomplete="off"` on input[type="checkbox"]

Not clear from the spec whether this is invalid per se.
But it is the current best known workaround for Firefox's unusual behavior.
Make warning from #13875 slightly more detailed

[skip sauce]
Merge pull request #14191 from twbs/autocomplete-off-btns

Use autocomplete="off" in buttons plugin docs examples