DDC-3175: Update documentation for QueryBuilder to give example of using "set()" with parameter

[doctrinebot]

Jira issue originally created by user max.summe:

QueryBuilder->set method uses Expr\Comparison object, which tries to cast values as strings.

When trying to set('updated', new \DateTime) in an update statement, this causes an exception as \DateTime has no **toString method.

Not sure what the fix is - the format for the \DateTime there depends on the platform and ( I think) the type of field.

[doctrinebot]

Comment created by stof:

you should not set a value directly in the DQL query. You should use the query parameters instead.

[doctrinebot]

Comment created by max.summe:

So you're saying it should not be used like this.

$qb = $em->createQueryBuilder();
$qb->update('User', 'u')
->set("u.field", "new value")
->where("u.field = :oldvalue")
->setParameter("oldvalue", "old value");

Instead, it should be:

$qb = $em->createQueryBuilder();
$qb->update('User', 'u')
->set("u.field", ":value")
->where("u.field = :oldvalue")
->setParameter("oldvalue", "old value")
->setParameter("value", "new value");

Is that correct?

If yes, it would be helpful to add an example to the documentation in this page: http://docs.doctrine-project.org/en/latest/reference/query-builder.html