The Oracle docs are not really the reference documentation for the open-source stuff, since illumos and Oracle Solaris have diverged quite a bit by now. Some things are implemented in different ways. OpenZFS in illumos has many features not in the closed oracle zfs.
The real reference docs are the man pages [1].
Depending on the illumos distribution there are more task-oriented guides for OmniOS [2], OpenIndiana [3] or the SmartOS Wiki [4].
In-depth information (driver development, MDB, DTrace) is written down in the "illumos books" [5].
These are not in danger from oracle.
Sadly a lot of good historical documentation is already gone since oracle broke all the sun.com links and not even archive.org has those. :(