No, we scratched that section. When we started with v3's docs we had a clear look at our previous documentation and came to the conclusion that we simply try to explain too much, with all kinds of background info, decisions made etc. While actually, people just want to know how to do ABC.
We also asked ourselves if our documentation should be the source of information about relational modeling concepts and the like, thus re-writing the works of Halpin, Chen, Yourdon and others. We came to the conclusion that we shouldn't do that. The philosophy is that if you don't know what entity inheritance is, you very much likely don't need/shouldn't use it.
Though, it might leave an empty spot in the range of elements to document. The v2.6 version of the inheritance concept documentation is focused on our own framework, and refers to NIAM/ORM which is now known by many people. So it wasn't really usable for v3, so if we add it, it has to be rewritten almost completely. As simple as it might sound, explaining this isn't easy, but you might have a point that it is a blind spot in our documentation.
So in short: it might be that in the attempt to bring a more 'do this, get that' kind of documentation, we missed a spot where we could inform the user/reader with better info. We'll look into adding a short concept documentation about inheritance mapping.
