Help tag rendered by #control_group method

This way is possible to render help and error messages for standalone control_groups.

Example:

```
<%= f.control_group :skill_level, label: { text: 'Skill' }, help: 'This is important!' do %>
  <%= f.radio_button :skill_level, 0, label: 'Novice', inline: true %>
  <%= f.radio_button :skill_level, 1, label: 'Intermediate', inline: true %>
  <%= f.radio_button :skill_level, 2, label: 'Advanced', inline: true %>
<% end %>
```