[Form] explain how to use `getOrigin()`

[alexislefebvre]

getOrigin() is mentioned in https://symfony.com/doc/5.4/components/form.html#accessing-form-errors

This documentation is evasive, how people should use getOrigin()? I tried and failed, see #17669

Source:

symfony-docs/components/form.rst

Lines 755 to 757 in fd19324

	// a FormErrorIterator instance in a flattened structure
	// use getOrigin() to determine the form causing the error
	$errors = $form->getErrors(true);

[xabbuh]

Maybe we should remove that sentence. I am not sure it really adds any value.