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!
The standard is "Prior to Symfony 2.6, this column did not exist."