This is a very subjective claim. Maybe a long long time ago you could say this, but not the case today.
> "not convinced that free software documentation is any better"
That's beside the point. If that were true, that is an argument for focusing a community effort at improving documentation for a free desktop. And actually with linux the problem is not as much a lack of documentation, but rather that the documentation is a little overwhelming, because it is just too specific or technical. (ever typed "man" something in a terminal?)