Fix hover-tooltip flickering when mouse re-enters

- is(':visible') seems to be the only reliable check, without a refactoring of how hoverState is used
- tests need improvement
Add section to docs explaining our stance on validators
Address #14541 by tweaking badge docs

* move unrelated .btn-primary example out of the pills subsection
* remove excessive .nav-stacked example
Merge pull request #14543 from twbs/validation-docs

Add section to docs explaining our stance on validators
Merge pull request #14546 from twbs/address-14541

Address #14541 by tweaking badge docs
Fixes #14545: Avoid strange formatting on vendor prefixing for placeholder
Merge pull request #14273 from sgonyea/master

Fix hover-popover/tooltip flickering when mouse re-enters