- Both classes do about the same thing, but with different names
- Clarifies docs for .m-x-auto requiring a fixed width block element for it to work
- Add missing heading for clearfix section in docs (unrelated)

Dump the type margin overrides in alerts for utils, document that particular example, sidesteps part of #18884

Closes #19049

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see https://github.com/twbs/bootstrap/pull/18782

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Put all the textual input types right next to each other for easy visual comparison.
Refs #17308, #18763

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- More consistent states and labels in nav examples
- Add stacked pills example for with <nav> element

Refs #18708
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It has been removed from the HTML specification: https://github.com/whatwg/html/issues/336
Only Presto Opera ever implemented a special UI for it.

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/fyi @mdo

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- add more details on custom checkboxes and radios for how we handle the states and how the css works
- create a disabled custom checkboxes and radios example

As per our browser compat policy, we only claim support for latest
versions of Opera on desktop. Current Opera is version 34. From testing, IE11 does support `pointer-events:none` just fine now.

to harmonise it with the callout given in pagination

"Go somewhere" is more descriptive of an example link, rather than a
button that triggers in-page functionality. Also makes it consistent
with all other examples in this page

...as per the text immediately preceding this example

It's unclear why the example currently has the content that's
shown/hidden BEFORE the toggle. Generally, this is not a recommended
content order, as after toggling, the newly shown content precedes the
toggle and requires keyboard users, and particularly assistive
technology users, to then navigate in reverse to reach it.

Based on v3 http://getbootstrap.com/css/#forms-help-text, adapted to
make examples more "real life"

Use new `.breadcrumb-item` class instead of child selectors and `li` tag selectors,
so as to no longer require the usage of `<ol>`-based markup.

Rename variables to follow naming conventions:
* $breadcrumb-padding-vertical => $breadcrumb-padding-y
* $breadcrumb-padding-horizontal => $breadcrumb-padding-x

Introduce new variable: $breadcrumb-item-padding

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as on first skimming over these, it may not be completely obvious

...otherwise, by the same logic, it should be "Bolded text" above. just
looks clunky...