Document how to render validation errors inside a dialog with a form

lookbook/docs/patterns/05-dialogs.md.erb

@@ -127,6 +127,29 @@ end
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 ```
 
+### Dialogs with form validations
+
+Sometimes a dialog with a form needs validations. If the form is submitted with turbo, the form can be rendered again with the validation errors within the dialog.
+In order for the dialog to not be closed, the rendering has to happen with an error response code (therefore 4xx or 5xx).
+
+```ruby
+class TestController < ApplicationControler
+  # include the helper module
+  include OpTurbo::DialogStreamHelper
+
+  # ...
+
+  def update
+    if @my_model.save
+      # ...
+    else
+      component = ComponentThatRendersAForm.new(my_model: @my_model)
+      update_via_turbo_stream(component: component, status: :bad_request)
+    end
+    respond_with_turbo_streams
+  end
+end
+```
 
 ## Special kinds of dialogs