[Session] PdoSessionHandler notice about data size

[Tobion]

If the session data doesn't fit in the data column, it might get truncated by the RDMS (at least mysql). So the data gets corrupted and also PHP session system ignores the data without giving a warning.

So the docs should add a note for this case that people can

• increase the column size (e.g. BLOB to MEDIUMBLOB) if they have huge session data
• enable STRICT SQL MODE in mysql so that such an error would be noticed

Based on symfony/symfony#14647