* Fix docs horizontal scrollbar in Components > Modal
* Add overflow to docs example class
* Drop the handleUpdate() variant
While I understand the variant covers the "get a particular plugin instance" case from http://getbootstrap.com/docs/4.0/getting-started/javascript/#programmatic-api it seems weird that we do it here but not everywhere else in the docs (randomly checking other pages, we seem to stick to the documented methods as written on those pages). As this isn't properly explained as is, and as it adds nothing (more of a preference for authors), it would be easier to just drop the variant and stick with the documented method as outlined later on in the same page (avoids having to go off on a "why should you use this instead of that" tangent here).
* Add .modal-dialog-centered for optional vertically cenetered modal
Fixes#23638
* adds modal-dialog-centered class to docs and removes margin to avoid generating a vertical scrolling
* mention limitations
* fix aria attr
* Add `width: 100%` to the .modal-content for the centered version. Adding it here to avoid adding another selector by limiting it to the centered modal modifier.
