minor symfony#5460 Finish symfony#5291: Bootstrap form theme and checkboxes (anacicconi, WouterJ)

This PR was merged into the 2.6 branch.

Discussion
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Finish symfony#5291: Bootstrap form theme and checkboxes

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6+
| Fixed tickets | symfony#4906

Replaces symfony#5291

Commits
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e495337 Rewrite note a bit
32b1060 Bootstrap form theme and checkboxes

Author: xabbuh

cookbook/form/form_customization.rst

@@ -114,6 +114,12 @@ fragment needed to render every part of a form:
   but the CSS classes applied are the ones used to display the forms horizontally
   (i.e. the label and the widget in the same row).
 
+.. caution::
+
+    When you use the Bootstrap form themes and render the fields manually,
+    calling ``form_label()`` for a checkbox/radio field doesn't show anything.
+    Due to Bootstrap internals, the label is already shown by ``form_widget()``.
+
 In the next section you will learn how to customize a theme by overriding
 some or all of its fragments.