More Robust 3.x to 4.x documentation

https://codeigniter4.github.io/CodeIgniter4/ is not the official User Guide.
It is the current in-progress User Guide.
If you will see it as the official one when the next version is released.

> If there are things missing, should I post them in this thread or create an issue in Github?

GitHub issue is better. It will definitely catch the attention of the maintainers.
But either will do.

That explains why I didn't see it... OK, I'll submit a github issue for anything not covered in there.