Document the server_version option of DoctrineBundle for DBAL

[stof]

DoctrineBundle 1.3 (compatible with Symfony 2.3+) added the possibility to configure the server_version option of DBAL 2.5.
This option needs to be documented because Doctrine ORM unfortunately triggers the retrieval of the database platform too early currently, and for drivers needing to change the platform based on the server version, this triggers an early connection to the DB in case it relies on guessing.
This is likely to concern lots of people upgrading to the new DBAL 2.5 given that MySQL is one of the platforms needing version switches

See doctrine/DoctrineBundle#351 (comment) for a config example.

[wouterj]

Hmm, so if I'm correct, there is a BC break in the Doctrine bundle which causes problems for many people (all people using MySQL for instance)?

[stof]

There is no BC break in DoctrineBundle. There is a change in DBAL, and because of the way the ORM uses DBAL, the version guessing is triggered even in case we don't actually need it, which requires connecting to the DB if the version is not explicitly configured.

But in any case, the new setting should be documented