[Components][Form] document $deep and $flatten of getErrors()

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#9918)
| Applies to    | 2.5+
| Fixed tickets | #3660

This is based on #4168.
I didn't try it, but can't we use `RecursiveTreeIterator`?