fixed : add doc blocks to silence Symfony deprecations

[encreinformatique]

Symfony 5.4 returning deprecations about

User Deprecated: Method "Twig\Extension\ExtensionInterface::getFunctions()" might add "array" as a native return type declaration in the future.
Do the same in implementation "Nelmio\ApiDocBundle\Render\Html\GetNelmioAsset" now to avoid errors or add an explicit @return annotation to suppress this message.

Closes #1921

[haidukua]

@encreinformatique
mb use native PHP types)

/**
* {@inheritdoc}
*/
public function getFunctions(): array
{}

[encreinformatique]

@encreinformatique mb use native PHP types)

@haidukua It was my first choice but I have doubts about child inheritance and PHP 7.1 (min package of this repo) as the parent function do not have type hinting.
I had in mind it was introduced in PHP 7.3. I'll check.

--
I got confused with property inheritance. All good for me then. Made the change.

[GuilhemN]

Nice, thank you @encreinformatique!