Explain form() and form_widget() in form customization

Continues https://github.com/symfony/symfony-docs/pull/4365

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | -

Original PR description:

> In the Form chapter of Book for Symfony 2.3, we use `{{ form(form) }}` to render the entire form ( in contrast to `{{ form_widget(form) }}` for older version of Symfony ). so maybe it should use `{{ form(form) }}` here for consistency.