At this point, this is unfortunately not possible, and is just a little helper text for the admin
One workaround would be to use jquery. Open up wrapper.html and find this
on a new line below we will target the field container and insert some text
Code:
$('#acf_myfield_container').append('<p class="help-block">my text here</p>');
adjust acf_myfield to match your field name and adjust the text