private void addDescriptionHtml(final DescriptionHTML descriptionHTML) {
        if (this.descriptionHTML != null) {
            throw new IllegalStateException("FormGroup can only contain one child DescriptionHTML widget. " +
                                            "Class: " + descriptionHTML.getClass().getName());
        }
        this.descriptionHTML = descriptionHTML;
        this.descriptionHTML.setStyleName("form-group-description");
        updateDescriptionPanel();
    }
Plain text, any HTML will be escaped.
The helpText attr trumps the {@code <form:HelpHTML>} element.
<p>
To programmatically set HTML help text, use {@link FormGroup#overrideHelpText(SafeHtml)}.
</p>
stroom-core-client-widget/src/main/java/stroom/widget/form/client/FormGroup.java