[Serializer] Add PropertyNormalizer::NORMALIZE_VISIBILITY documentation

[alamirault]

Try documenting #17036

[javiereguiluz]

Thanks Antoine.

Please note that while merging we reworded your contribution. The problem was that adding all new contents inside a versionadded directive is "dangerous", because in future Symfony version we'll remove all these versionadded directives and if we are not careful, we could lose this information.

Here's the reword: c0e8d75 Thanks!

[alamirault]

Thank you @javiereguiluz for your explanations !