Merge pull request #18540 from andreynering/guides-custom-form-builder

Improving 'Customizing Form Builder' section with example [ci skip]

guides/source/form_helpers.md

@@ -687,7 +687,14 @@ class LabellingFormBuilder < ActionView::Helpers::FormBuilder
 end
 ```
 
-If you reuse this frequently you could define a `labeled_form_for` helper that automatically applies the `builder: LabellingFormBuilder` option.
+If you reuse this frequently you could define a `labeled_form_for` helper that automatically applies the `builder: LabellingFormBuilder` option:
+
+```ruby
+def labeled_form_for(record, options = {}, &block)
+  options.merge! builder: LabellingFormBuilder
+  form_for record, options, &block
+end
+```
 
 The form builder used also determines what happens when you do