That problem is alleviated by the fact that, for libraries, updates to documentation are commits as well, so even a very stable piece of software still needs the occasional commit to update a FAQ or somesuch. All in all, I've been surprised how well activity meters work when vetting software.
Even that doesn't always apply. The amount of documentation available for my favorite window manager on the (ancient) site for it is mind-boggling. The creator labels the documentation as "comprehensive" and, to quote Bill: "Baby, you ain't kidding."
For most software, though, it's true that documentation can always use improvement.
