Documentation testing falls under the category of non-functional testing. Documentation Testing involves testing of the documented artifacts that are usually developed before or during the testing of Software. This is significant part to test since last minute fixes usually change the behavior of the system in a way, which serves from documentation.
Documentation for Software testing helps in estimating the testing effort required, test coverage, requirement tracking/tracing, etc.
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’s end user documentation. incorrect or poorly written or even missing documentation irritates the end users that tend to quickly form an opinion about the product. Hence quality of documentation bears a clear reflection on the quality of the product as well as that of the supplier who supplied it.