Perhaps it's not as large as you think, relatively speaking. If we take a look at bugs.freebsd.org, it seems that many bug reports are opened and left to rot for years. The documentation section of the tracker is no exception.
Judging by the massive amount of unfixed bug reports linked in this email, doesn't it seem like documentation has been an afterthought for them for a long time?
Does anyone have a comparison for OpenBSD or NetBSD's documentation having outdated or incorrect info?
https://bugs.freebsd.org/bugzilla/page.cgi?id=showreport.htm...
Seems to be getting worse with time too.