But... I keep being told that commit messages are useless because the code is the documentation, so code diffs are self-explanatory...
∞ Try make the code sensible & readable so it can be the documentation.
∞ Comment well anyway, just in case it isn't as obvious to the reader (which might be me in a few months time) as it is to me when making the change. Excess comments can always be removed later (and, unless some idiot rewrites history, can potentially be referred to after removal if you have a “why t f” moment), comments you never write can't be found later.
∞ Either a directly meaningful commit message, or at very least ticket references to where more details can be found.
For personal tinkering, I'm a lot less fastidious.
It never actually is at any non-minimal scale (and not even the code authored by the the people who claim code is self documenting).
My comment was rhetorical and sarcastic.
/s
When the "why" isn't explained, you end up with things like someone refactoring code and spending time (at best) trying to figure out why some tests now fail or (at worst) breaking something in production.
I'd argue that even the "how" sometimes is better explained in plain words than in code (even if that opens the door for outdated comments when code is changed).