Fix docs horizontal scrollbar (#24878)

* 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).

assets/scss/_component-examples.scss

@@ -118,6 +118,7 @@
   position: relative;
   padding: 1rem;
   margin: 1rem (-$grid-gutter-width / 2);
+  overflow: auto;
   border: solid #f7f7f9;
   border-width: .2rem 0 0;
   @include clearfix();

docs/4.0/components/modal.md

@@ -459,7 +459,7 @@ For modals that simply appear rather than fade in to view, remove the `.fade` cl
 
 ### Dynamic heights
 
-If the height of a modal changes while it is open, you should call `$('#myModal').data('bs.modal').handleUpdate()` or `$('#myModal').modal('handleUpdate')` to readjust the modal's position in case a scrollbar appears.
+If the height of a modal changes while it is open, you should call `$('#myModal').modal('handleUpdate')` to readjust the modal's position in case a scrollbar appears.
 
 ### Accessibility