Document limitations of buttons JS plugin

Addresses #14148.
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]
Add Wall of Browser Bugs entry for IE bug from issue #14211

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

Use autocomplete="off" in buttons plugin docs examples
Merge pull request #14188 from twbs/doc-14148

Document limitations of buttons JS plugin
Merge pull request #14225 from twbs/issue-14211

Add Wall of Browser Bugs entry for IE bug from issue #14211