Whenever I start trying to learn a new codebase, one of the first things I do is try to get an understanding of how the code works at a high level. Generally this isn't too hard, I just find out where the program starts, which is usually pretty obvious, and work through the code from there. But there's a problem I always run into the code only tells me what it does, it doesn't tell me the most important thing: why. Unfortunately, most projects don't maintain a good set of code-level documentation. Instead all documentation is locked away inside of heads of its developers as tribal knowledge.
So if documentation is needed where's the best place to keep it? In my opinion, the the documentation needs to be as close to the code as possible, so put your documentation in the code. The further away it is, the more likely it is that the documentation will get out of date, and the only thing worse than no documentation, is wrong documentation.
Now, what should be in those comments? Well, as I eluded to previously, it should tell me the why. If someone is reading through the code they surely have enough knowledge to be able to tell what's happening to which objects, so you can safely leave that out.
