[#3276] Trying to further clarify the session storage directory details

Hi guys!

This continues after #3342. Basically, it's confusing the symfony/symfony has a different default than the Symfony SE. However, I agree with Wouter that most framework readers are probably SE users. My strategy is simply to show people which configuration causes which situation.


| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.3+)
| Fixed tickets | #3276 

Thanks!