[Intl] IntlDateFormatter is abstract, yet it promotes instantiation

**Symfony version(s) affected**: 4.4.11

**Description**  
In https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Intl/DateFormatter/IntlDateFormatter.php#L24-L27 it promotes creation, but it's abstract.

**Possible Solution**  
Update docs.
