feature #12464 [Form] Documented "Supporting Bootstrap 4 custom switches" (romaricdrigon)

This PR was merged into the 4.4 branch.

Discussion
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[Form] Documented "Supporting Bootstrap 4 custom switches"

Hello,

This PR add the doc for the corresponding proposed feature, custom switch support in default form theme:

![image](https://user-images.githubusercontent.com/919405/66652902-1919fa00-ec37-11e9-98f3-9340b01b2335.png)

See the Symfony PR for implementation details.

Commits
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fbfeec1 Documentation for Bootstrap 4 custom switches

Author: wouterj

form/bootstrap4.rst

@@ -92,13 +92,17 @@ Custom Forms
 ------------
 
 Bootstrap 4 has a feature called "`custom forms`_". You can enable that on your
-Symfony Form ``RadioType`` and ``CheckboxType`` by adding a class called ``radio-custom``
-and ``checkbox-custom`` respectively.
+Symfony Form ``RadioType`` and ``CheckboxType`` by adding some classes over the label:
+
+- for radio, use ``radio-custom``
+- for a custom checkbox, use ``checkbox-custom``
+- for having a switch instead of a checkbox, use ``switch-custom``
 
 .. code-block:: twig
 
     {{ form_row(form.myRadio, {label_attr: {class: 'radio-custom'} }) }}
     {{ form_row(form.myCheckbox, {label_attr: {class: 'checkbox-custom'} }) }}
+    {{ form_row(form.myCheckbox, {label_attr: {class: 'switch-custom'} }) }}
 
 Labels and Errors
 -----------------