In my case, yes, the system was exceedingly well documented, and also made use of standard tools (Makefiles, perl and bash scripts, etc.).
But I don't think documentation is a panacea if the tool used is particularly rarified. Perhaps the DBA in question (this is purely hypothetical now) set something up using Oracle, and then left. Now, maybe it's easy enough to use as the interface for SQL queries and the like, but what happens if there are major reorganizations that really do require specialized knowledge? Can you document all possible contingencies? Without simply giving enough learning materials for the user to become a DBA? (I have no idea, honestly, since I don't have any experience with high-end DBs, I can't say anything about how hard it is to maintain one, so I'm more making a general point rather than a specific one.)