That is an expression mostly heard from students at schoool.

There are huge amounts of documentation available out there for Linux itself, the different subsystems etc. A large number of open-source projects have specific people involved in the documentation of the software. People who do want the documentation to be good, because their name is shown in the documentation.

The truth is that in many situations you get good documentation and the possibility to compare documentation with source code. Lots of companies have documentation that is incorrect (but beautiful) and you can't verify the claims in the documentation. All you can do is notice that the claims aren't true on your specific environment. Then you have to fight with a support division to get a bug fix, unless the feature in the documentation wasn't even implemented - maybe just a nice expression added to the documentation just because the sales department (who lack the technical skill) thought it sounded nicely.

It really is a big issue that many commercial projects have documentation that makes incorrect claims just because of marketing. They want the customer to fork out money up-front. So the manual is part of the sales material.

An important thing here is that lots of material available on the net are nothing more than the work of one or two (sometimes not so clever) students. Just as we can also often see students post (often not so clever) source code when other students visits forums and asks for help. But then it's the users responsibility to make some form of quality control of code found on the net before starting to use it.