There is always a reason why things are 1) done and 2) done in this specific way and not another. It is also relevant if this is part of a larger change and it there are external dependencies on this commit. These things must be clear from the commit message.
> you're just doing janitor work
That makes it sound like janitorial work does not require complete commit messages which is why we sometimes see useless messages. There is always a reason why something was done and this can be documented.
A bug can be documented why it happened, why it was fixed in this specific way and not some other more obvious way, and what steps has been taken not avoid it from resurfacing. The same way a clean up can describe why others paths were not taken, why the new way is considered more clean (fewer lines, lesser complexity?) and how we know it does not entail functional changes.
The git tool is rooted in email, and commit messages look a lot like it. Pretend you are describing your patch over email to someone and it will be almost trivial to write.