Support setting `help-block` either by facet or attribute.

[vsvetoslavov]

Bootstrap supports adding a help text to form controls. https://getbootstrap.com/docs/3.3/css/#forms

  <div class="form-group">
    <label for="exampleInputFile">File input</label>
    <input type="file" id="exampleInputFile">
    <p class="help-block">Example block-level help text here.</p>
  </div>

It would be nice to be able to define this help-block part - as a facet, as some attribute... To me facet seems more appropriate since it can contain html. Best regards!