What documentation does a distro even publish? I only ask because freebsd has the most solid documentation I've used of any OS I've ever encountered. I seriously doubt Debian has documented the linux kernel that well (which, tbf, would be an insane project to even attempt)
Debian has an installation guide[1]. I'd imagine all the major distributions do.
But it probably has to change a lot for every major release, because so many things change. FreeBSD major releases have changes too, but a lot of the user interfaces are very stable and so the documentation can be too. Stable documentation allows time for it to be edited and revised to become better documentation, as well as developing quality translations.
[1] https://www.debian.org/releases/stable/amd64/
I've been surprised at times by what's missing. There are are some strange omissions, from most recent memory around the Bourne shell builtin commands and make(1). I've had to go hunting in other BSD distribution manuals at times to find what I need. I'd say the GNU core utilities sometimes have better docs for their equivalent commands.
That said, for non-core utilities on Linux it's pretty hit-or-miss. The BSDs are generally pretty consistent in what they do offer, and that's what I love about them. Of course it's a different development model and it shows.