* Move helpful code comments

* Add .carousel-fade option to Sass

* Document example of the fade carousel

* more logical warning of the .active class on carousel items

* Add .dropdown-item-text class

* Add example of plain text within a .dropdown-menu, too

* reorganize, simple/clearer headings

Opera 19 was released in January 2014 and auto-updates so I suggest no need to mention about browser support for this old version.

Removed '**Animated progress bars don't work in Opera 12**—as they don't support CSS3 animations.' note.

Bootstrap 4 doesn't (and doesn't need to) support the ancient Opera 12 released in 2012 with a 0.04% marketshare so no need to mention.

* Make dropright snippet the same as demo

* Add missing .dropdown-toggle-split class

* Make clear the split button label the same as demo

fixes #25288

* Add additional form validation examples

Fixes #24811.

This adds .valid-feedback to our custom styles and server side examples; previously we ommitted this to suggest you don't always need valid feedback. In addition, this adds examples of the .{valid|invalid}-tooltip classes with a new subsection in the Validation docs.

* Update validation tooltip styles to remove fixed width; instead should retain itself to the parent element

* update ids

* finish docs paragraph, mention position: relative

* Remove incorrect roles from accordion, tweak/expand accessibility info
* Use `<button>` elements rather than links for the accordion

This commit allows nested structures for accordions. Also a part of
the documentation about data-children is removed because this
functionality didn't work and it's not applicable anymore.

Tests with the collapse accordion are also a bit adjusted to the new
situation.

"Choose file ..." text is now written inside the label tag, so the translation is outside the css scope. 
$custom-file-text variable should only contain "Browse" translations.

Closes #25067

- Changes the wrapping label to a div so we can style the label instead of another element while also supporting form validation.
- Fixes form validation styles for custom file input (closes #24831).
- Updates docs with validation styles (also adding example feedback text while I was there) and new how it works section.

Adds some additional context on `.form-group` usage

* Restyle code element, clean up variables

- Removes the padding and background-color
- Changes the color to a brand color ()
- Add new variables to kbd element styles to account for removing the code ones
- Remove overrides that were needed previously

* only break between words, and override it in the pre

* make file inputs 100% wide

* scope custom file changes in input group for sizing

* shorter button labels for responsive

* narrower cards

* button was too wide

* downsize

* fewer links in pagination

* redo cell text

* improve overflow docs

* match layout behaviors

* ditch the indicator as separate element for psuedo-elements on the label

* move disabled to attribute only on input

* redo default inline check to support new markup

* redo inline forms

* clean up vars

* update validation mixin to new structure

* update checks in docs

* linting for for/id attributes

* Rewrite input group component

* Set the feedback to 100% width for input group

* Move from .row to .form-row for tighter layout

* no need for custom feedback here, we're using browser messaging

* add input group to validation examples

* add note about validating multiple

* migration note added