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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...

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Closes #17291 by finishing the last remaining task.
Also reorders the sections in the "Download" docs.

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Should close https://github.com/twbs/bootstrap/issues/18556

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clarify lack of support for striped and animated striped bars in
IE10+/Edge

If these are supported, why bother mentioning them in a section that's about communicating what's not supported?
These rows made sense when the section also covered IE8 (http://getbootstrap.com/getting-started/#support-ie8-ie9 ),
which doesn't support these features, but we no longer care about IE8 in v4.

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https://connect.microsoft.com/IE/feedback/details/2110930/edge-ie11-gradient-background-image-on-tr-is-only-applied-to-first-td

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Fixes #18505
Closes #18509

Should close https://github.com/twbs/bootstrap/issues/18503

/fyi @mdo There might be prettier/better ways of explaining these,
        but for the sake of having accurate docs for tomorrow,
        this should do in a pinch.

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Refs #18110

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