Add example about FlashBagAwareSessionInterface

[VincentLanglet]

This seems enough to close #16934 to me

[javiereguiluz]

@VincentLanglet thanks for this contribution. However, after talking internally about this with the Symfony Core Team, I've decided to close this without merging it.

I've been told that the problem was that SessionInterface didn't have the methods that the Session class has to deal with flash messages. This is not important for most people because Session is the only implementation of SessionInterface. But for folks creating their own session implementations, it might be important.

So, your code changes are perfect and will help e.g. to fix "errors" reported by PHPStan ... but in practice, this is not really needed by most Symfony users. That's why I think it's better to not display these details in the docs. Thanks for understanding 🙏

[VincentLanglet]

No problem !