[Components][Form] describe how to access form errors

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

The ``getErrors()`` and ``getErrorsAsString()`` methods haven't been documented before. This pull request adds a short description for them. This forms as basis for the solution of #3660.
@xabbuh I see that you put #4169 on hold for this, so I came back. I think we should basically expand your code block to show 1, 3 and 4 in my above note, with a comment for each that says what they do:

```php
// an array of FormError objects, but only errors attached to this form level (e.g. "global errors)
$errors = $form->getErrors();

// an array of FormError objects, but only errors attached to the "firstName" field
$errors = $form['firstName']->getErrors();

// ... etc
```