@anonymous_user_7db7de02 your answer is the best, I think, but this is a very recurrent question since always. A quick search over the forum/answer-hub will lead to them.
In my humble opinion, there is no need of documentation when you have source code from a software, not only UE and yes I know is hard and it is not a 1:1 example, like everybody wants, but it is a powerful tool for learning. A more relevant question should be, how to read the source code? don’t you think?