It answers why the documentation is incomplete. It’s a literal disclaimer that basically says “this documentation is incomplete.” I get the question was “where?”, but that assumes Epic maintains a second “real” documentation that’s hidden somewhere, which is very unlikely since that would be more work than just maintaining one documentation.
Agree with this. If something doesn’t have documentation, it doesn’t just make it unusable. When I started digging into source code, I used the documentation for a lot of things. When it was missing something, I literally just used… the source code. 
The source will likely have comments, so there’s your documentation right there.
The engine’s source code is supplied for convenience, not necessity; it’s meant to be modified & extended, but not necessarily dissected. If you don’t know what something does, just experiment with it. The worst you can do is crash the program.
I made it a habit to comment every block of code I write as I write it, as if it were part of the code itself. Now, it’s hard for me not to comment my code; it looks “incomplete” to me if I don’t. I know comments and documentation are not necessarily the same, but an easy way to make documentation is just to use the comments.
You misunderstood him: