Correcting the doc because mysqli doesn't support named parameter natively

The documentation incorrectly stated that their use was possible.
If I do that, on [that documentation page|http://www.doctrine-project.org/api/dbal/2.2/class-Doctrine.DBAL.Driver.Mysqli.MysqliStatement.html#_bindValue] it's still going to state that it support the named parameter which is wrong.

On other solution is to change the main definition to state that the named parameter is only supporterd natively on some driver.
What I could do is add the