* Past incidents
* Regulations
* What the code used to do, and why that was bad
The whole point of a really good comment is that you can't infer it just by looking at the code. That's why it's there at all!
Yes they become paragraph long. But that's the point. That's where I search for "why". And not just "why", "when, why and with what reasoning?".
Imagine if there were some interference that could derive a comment based on history and current code!
I'm certainly a fan of including lots of detail in commit messages, but often I do so and then realize "hey, this particular information would be better situated close to the code", and I move it.
Other times, yes, it's really just best to leave in the commit message because no one looking at the after-version of the code will care. For example, removal of a deprecated feature.