Symfony FormBuilder - 附加元素的自定义选项？

I've worked out my own solution...

I added a 'help' attribute to the required fields. E.g.

->add('username', TextType::class, array(
    'attr' => array(
        'help' => '<p>Your username will be used as a way of <em>uniquely</em> identifying you when you log into your account. You can still log in using your email address in case you forget your username.</p>
<p>It must be between 2-25 characters long and start with a letter. The following characters are permitted: letter, number, dot, underscore, dash.</p>'
    ),
    )
)

I then had to overwrite the form_row block to add my help/info box and contents. Note the use of the 'raw' filter - I needed this as my help text includes html tags that need to be rendered so using this filter the tags will not be escaped.

{# \app\Resources\views\form\form_div_layout.html.twig #}
{% block form_row %}
  {% spaceless %}
    <div class="form-group">
      {{ form_label(form) }}
      <div class="col-sm-10">
        {{ form_widget(form) }}
        {{ form_errors(form) }}
        {% for attrname,attrvalue in attr %}
          {% if attrname == 'help' %}
            <div id="info-{{ id }}" class="additional-info alert alert-info alert-dismissible">
              <button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">&times;</span></button>
              {{ attrvalue | raw }}
            </div>
          {% endif %}
        {% endfor %}
      </div>
    </div>
  {% endspaceless %}
{% endblock form_row %}

This got the info box to display correctly. However, it left the original text in a 'help' attribute of the field, so I removed that by overwriting the widget_attributes block and ignored outputting the 'help' attribute.

{# \app\Resources\views\form\form_div_layout.html.twig #}
{%- block widget_attributes -%}
    id="{{ id }}" name="{{ full_name }}"
    {%- if disabled %} disabled="disabled"{% endif -%}
    {%- if required %} required="required"{% endif -%}
    {%- for attrname, attrvalue in attr -%}
        {%- if attrname == 'help' -%}
          {# Prevent output of the help attribute #}
        {%- else -%}
          {{- " " -}}
          {%- if attrname in ['placeholder', 'title'] -%}
              {{- attrname }}="{{ translation_domain is same as(false) ? attrvalue : attrvalue|trans({}, translation_domain) }}"
          {%- elseif attrvalue is same as(true) -%}
              {{- attrname }}="{{ attrname }}"
          {%- elseif attrvalue is not same as(false) -%}
              {{- attrname }}="{{ attrvalue }}"
          {%- endif -%}
        {%- endif -%}
    {%- endfor -%}
{%- endblock widget_attributes -%}