Button groups for checkboxes/radio buttons needs to be documented

[scurker]

The style for checkboxes/radio buttons in button groups is currently already in buttons.scss but needs to be fully documented.

Example Markup

<div class="btn-group">
  <input id="class-level-anyone" type="radio" name="level" value="anyone" checked />
  <label for="class-level-anyone" class="btn">Anyone</label>
  <input id="class-level-intermediate" type="radio" name="level" value="intermediate" />
  <label for="class-level-intermediate" class="btn">Intermediate</label>
  <input id="class-level-advanced" type="radio" name="level" value="advanced" />
  <label for="class-level-advanced" class="btn">Advanced</label>
</div>