Add a mention of an addition of nullable type declarations

Author: dabrt

docs/release_notes/ibexa_dxp_v5.0.md

@@ -387,6 +387,7 @@ The following Twig functions have been added in the v5.0 release:
 This release brings other minor upgrades intended to improve the developer's experience:
 
 - To improve code clarity, reliability, and error detection, type hint declarations that specify the expected data type have been added in multiple places throughout the product
+- In an anticipation of [changes coming with PHP 8.4](https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated), implicit nullable type declarations have been replaced with nullable type declarations throughout the product code. It is recommended that you update your custom code in the same way
 - Developer experience has improved with capabilities offered by PHP in version 8.3. For example, the `AsTwigComponent` attribute [facilitates autoconfiguration](components.md#php-code) of Twig components
 - With protection against breaking changes and easier refactoring in mind, [TypeScript](https://www.typescriptlang.org/) can now be used to extend the Back Office
 - [[[= product_name_base =]] Rector package](https://github.com/ibexa/rector) has been introduced that is based on [Rector](https://github.com/rectorphp) and comes with additional rules for working with Ibexa code. You can use it to get rid of PHP code deprecations