- Update docs examples to use newer .form-check suite of classes
- Update form validation mixin to use newer classes as well

Remember, there may be children/students using these examples...

- 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

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

- 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.

Uses a new more specific class, adds some basic spacing, and clarifies
that this class is only for validation feedback.

Clears up the help text class question in
https://github.com/twbs/bootstrap/pull/18704#issuecomment-168241731 and
addresses the vertical spacing in #18703.

- Add container to fix padding in mobile views
- Add large and small variations
- Cross link grid classes and mixins
- Fix up some words

Alternate fix to #17965: Updates to use the new class name and adds some basic docs for custom checkbox/radio validation

Fixup for 365db6e3
/fyi @mdo
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Closes #19049

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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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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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- 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

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

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Reunites the family.
Closes #17791