I think that is a documentation issue, in http://www.doctrine-project.org/docs/mongodb_odm/1.0/en/reference/query-builder-api.html (15.3. Modifier Operations) the queries are like:

$dm->createQueryBuilder('User')

>field('password')>set('newpassword')

>field('username')>equals('jwage')

->getQuery()
->execute();

But it is missing the update(), it should be like:

$dm->createQueryBuilder('User')

->update()
>field('password')>set('newpassword')

>field('username')>equals('jwage')

->getQuery()
->execute();

All the queries in that section have similar issue