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">×</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 -%}