Example docs usage of `form-check` requires `form-group`

Created by: peterblazejewicz

Hello,

I'm under impression the sample form markup in the docs needs a correction here:

http://getbootstrap.com/docs/4.0/components/forms/

<div class="form-check">
    <input type="checkbox" class="form-check-input" id="exampleCheck1">
    <label class="form-check-label" for="exampleCheck1">Check me out</label>
</div>

The problem with sample snippet is that AFAIK the form-check improves rendering of custom checkbox control but not a layout within a form. So I think it should be supported by a form-group to both layout a group and customer control:

<div class="from-group form-check">
    <input type="checkbox" class="form-check-input" id="exampleCheck1">
    <label class="form-check-label" for="exampleCheck1">Check me out</label>
</div>

Sample in CodePen: https://codepen.io/blazejewicz/pen/YeqEPR

Thanks!

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