As another seasoned coder, I’m just used to exactly this kind of thing - the only really well documented hardware I’ve used was back in the C64 and Amiga days where we had ROM kernel reference manuals and hardware reference manuals etc - of course, that was only possible because the ROM was ROM and the hardware was propriety.
Things are too dynamic, Epic could stop development and focus on catching all the bugs introduced from coders working on the engine (some Epic, mainly focused on UEFN and some devoted devs submitting PRs) and spend the months and months of documenting it well (the reference pages are just generated by comments in the code by the looks of it) - but then the engine gets left behind from hardware and algorithm evolution - which is going through a growth spurt rn.
I think if this is becoming an issue for you, maybe build from source and modify the code yourself to fix anything that’s not working, don’t get tempted to install the latest version, or just use it as reference. Most studios stick to a version and modify that anyway.
The Unreal Source discord channel can be very useful, there’s a lot of very talented, intelligent and experienced people there that are happy to help - I’d actually go as far as saying that’s more useful than detailed documentation.