Replaced configuration documentation with a link to the up to date config documentatio...

...n in the Symfony docs

There are two versions of the DoctrineBundle documentation with different information.

https://github.com/symfony/symfony-docs/blob/master/reference/configuration/doctrine.rst
https://github.com/doctrine/DoctrineBundle/blob/master/Resources/doc/configuration.rst

The one in the symfony-docs is kept up to date so I added a link to them. 

Please see for more information the issue I made in the symfony-docs repo: https://github.com/symfony/symfony-docs/issues/4037
@stof I changed the link to the rendered doc. Thank you for the review.