Documentation testing assures that the written explanations of how to use the system practically match what the system does.
This is significant part to test since last minute fixes usually change the behavior of the system in a way, which swerves from documentation.
A good software tester should know that all kinds of user documentation must be tested: this comprises operational guides as well as the usual application end user documentation.
Documentation can be tested by asking users try to use the new system using the documentation provided.
After this you should choose those users who are conversant with their own domain (operation, application, administration and so on) but who are not conversant with the new system.