In reference guide, 12: DQL http://www.doctrine-project.org/docs/orm/2.0/en/reference/dql-doctrine-query-language.html
There is a part for partial object syntax: 18.104.22.168. Partial Object Syntax http://www.doctrine-project.org/docs/orm/2.0/en/reference/dql-doctrine-query-language.html#partial-object-syntax
It should have some reference to 16. Partial Objects chapter as it gives some detailed explanation about the subject and has important notes. http://www.doctrine-project.org/docs/orm/2.0/en/reference/partial-objects.html
Also in chapter 16 it says partial objects are not allowed by default. There is no mention to that in chapter 12 part.