added docs about child forms customization

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.2+
| Fixed tickets | none

I was not aware that you could apply a form theme to a specific child until I asked Bernhard on twitter, and thought it could be useful to someone.
This is my first PR to symfony-docs, I hope it'll do.