Everything is in one place, and it is maintained. In contrast, Linux requires you to go look up documentation in a lot of different places, and it is highly dependent on distribution what to do in a given situation.
One reason for this is that FreeBSD maintains not only a kernel, but also a userland. This means a change in the kernel can immediately be followed by appropriate changes in the userland and vice versa.
But my experience with FreeBSD's docs are even better - and most of that experience is quite old at this point. I can't imagine how much better the situation has gotten since FreeBSD only seems to mature and not degrade (based on comments like yours which are very common).
I also seriously dislike the BSD's tendency towards making --help useless, forcing you to bring up the man page. This is just for practical reasons. It's easier to hit up arrow, dash dash help pipe less than up arrow, ctrl+a, man, alt+f, ctrl+k to kill the arguments you've already written :)
Man pages in OpenBSD for instance are a "must". In the GNU world, often man pages are really badly maintained pointing to GNU info. Also, the so called "Linux Howtos" were a disaster full of obsolete points.
Just compare the OpenBSDxy.html upgrading page (where m and n are release numbers) for OpenBSD releases (and the FAQ) to the Arch Wiki.
OpenBSD also strikes a very good balance between "informative" and "terse" that should be held up as a model for others. Unfamiliar things may need a few readings to get everything, but it's all there without being a toilet-clogger.
/me adds a new project to the backlog...
With NetBSD and OpenBSD everything looked sane. They explained you everything, from /etc/profile, managing services without making you mad with runlevels, to launching X and so on.
twitch
Small English correction: the noun directly after the verb "to explain" must be the thing that is being explained. What you wrote means "they explained [anthk] [to everything]". They gave an explanation of you to the whole universe?
This is a common mistake but it winds me up every time.
"They explained" + the thing they explained + optionally to whom or what they explained it.
It should be: "they explained everything to you."