[Validator] Improve documentation for UniqueEntity constraint

[ker0x]

Replace #14458 and #20166

I'm targeting 7.3 because i don't know if changes made in 7.1 are transferred to 7.3. Otherwise, let me know and I'll change it.

[wkania]

Nice. Looking at my pull request, I think it would also be helpful to mention which version of Symfony introduced this feature.

.. versionadded:: 7.1
    Any class instance (like DTO) field (or fields) validation against entities persisted in the database was introduced in Symfony 7.1.

.. versionadded:: 7.1

    The option was introduced in Symfony 7.1.

[wkania]

The type should be **type**: ``array`` | ``string`` [:ref:`default option <validation-default-option>`]
It accept string when not combined identifier. Like:
identifierFieldNames: 'id'

[ker0x]

I don't think so because the property is typed with ?array in the UniqueEntity class: https://github.com/symfony/doctrine-bridge/blob/7.3/Validator/Constraints/UniqueEntity.php#L60

[wkania]

Now, I see that in the end I didn't do it like fields argument.