Pavel,
To me, what was most helpful in the old documentation was the architectural discussions embedded in the enhancement requests. When the requests were satisfied.
Examples:
use of short names in non-local domains
auto-discovery of trusted domains
For instance, until I read that discussion I was unaware that if you turned off auto-discovery and explicitly defined each child domain, that sssd would no longer contact the global catalog (GC) and thus, full membership in universal groups was not found. Only if you turned on auto-discovery did the sssd code query the GC. this is not intuitive, so having that documentation was a great help.
Another example is the discussion of that better discovery algorithm for AD DCs. It came out in a recent sssd release, maybe in the last 6-9 months. But I can't find that algorithm discussion now.
Spike