Adding details about the changes to the PdoSessionHandler in 2.6

Hi guys!

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes symfony/symfony#12833
| Applies to    | 2.6+
| Fixed tickets | n/a

This follows #4557.

This adds more details about the table schema changes to the PdoSessionHandler in 2.6. I also made some improvements (not showing all the extra configuration by default), which I'll backport to 2.3 after this is accepted.

Thanks!
I think we could in theory, but it would take a bit of work (reading the config) + reading the DB engine. And it would be a little risky, since an upgrade script implies that it'll work (I could see people running it and expecting with 100% faith that it'll work, when maybe they've made some additional session changes). It's an unfortunate BC break, so I would like to make this change as obvious and easy as possible... but not too easy :)