Clarify valid values of Carousel's `pause` option

Fixes #18205

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Fixes #17193: Remove mention and link to non-existant transitions plugin; that intro paragraph was two dupe sentences

Since the "remote" option is no longer available in v4 the reference to the "loaded.bs.modal" event is no longer needed, too.

- Really we didn't need to do any of that strip units stuff to generate six new variables, two for each button size
- Using sibling selectors, we can target those split button dropdown toggles using the .dropdown-toggle-split class, and adjust padding and margin as needed
- Now, we nuke the margin-left from the ::after generated caret and tighten up the padding so that those split toggles don't look huge next to their main button

- Includes new class
- New docs to support it
- Rearranged older docs

- Add lighter gray text to disabled state for .form-check
- Separate radios and checkboxes example
- Clarify docs

- Move disabled radio and checkbox styles to Reboot
- Collapse .radio and .checkbox into single class, .form-check
- Collapse .radio-inline and .checkbox-inline into single class, .form-check-inline
- Require classes for sub-elements in both new classes

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Fixes #18573: Change name of .form-control-label to .col-form-label (and .form-control-legend to .col-form-legend) to reiterate use for only grid layouts

Refs #19665
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The modal backdrop positioning bug related to iOS' virtual keyboard doesn't seem to reproduce in iOS 8.0+.
(Possibly as a side-effect of https://bugs.webkit.org/show_bug.cgi?id=153224 )
Refs #9023
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Fixes #18493
Closes #18836
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Add documentation to components/utilities to publicly expose the
.pos-f-t utility class to users.

Closes #18799
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It's very handy to add another spacer size (e.g. `4`) which can easily be done by extending the map.
Example:
```
$spacers: (
  0: (
    x:   0,
    y:   0
  ),
  1: (
    x:   $spacer-x,
    y:   $spacer-y
  ),
  2: (
    x:   ($spacer-x * 1.5),
    y:   ($spacer-y * 1.5)
  ),
  3: (
    x:   ($spacer-x * 3),
    y:   ($spacer-y * 3)
  ),
  4: (
    x:   ($spacer-x * 5),
    y:   ($spacer-y * 5)
  )
);
```

Closes #18706
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- Add/replace callout for color and AT with include

This one was overlooked at edit

See #17719 and #19199

- Give some guidance on when someone might use any of the three
validation states. Fixes #18702.

- Improve the examples to provide examples of supporting validation
text with the new `.form-control-feedback`, as well as always-present
help text. Nullifies #18704.