It's interesting to note that in the case of GNAT, we have no licensing constraints on the documentation that would restrict automatic generation, but we just don't do it.
The GNAT documentation is pretty complete, and certainly gets a lot of attention and constant improvement, since we regard it as being as important as the code. In the case of interfaces to library routines, what we do is to have fully commented Ada package specs that act as both the documentation of the implementation interface and as the user documentation (for an example, look at g-spipat.ads). I can't see any value in duplicating this information elsewhere.
