[Doctrine] Add precision for doctrine lifecycle callbacks

[duboiss]

I think it's a good idea to specify that the methods must be public.

• If the developer inadvertently makes the callable private, they will just realize that the method is not executed. No exceptions thrown.
• By rereading the doc, it would be simpler for the word "public" to catch their attention.

[javiereguiluz]

I agree! This will make things easier to understand and will avoid confusion for some people. Thanks Steven and congrats on your first Symfony Docs contribution 🎉