[Form] [BestPractice] Forms as services section outdated

[backbone87]

https://symfony.com/doc/current/best_practices/forms.html#registering-forms-as-services

The reasoning given in this section is outdated.
Since the change to form type handling (since 2.8 iirc), i think the only reason left to register your form types as services is when you want to inject some dependencies into it.

[wouterj]

I agree with your opinion and proposal. Can you maybe make a PR with this change by clicking on the "edit" button of that page?