I tend to keep it simple - the offense, number of reminders they have. The more you write, the harder it is to skim through later and get a quick and clear idea of what the user is like.
I would always just write notes for future moderators to reference and decide what action to take. For instance, if we had one user that was a repeat offender, I would write the moderation action I took, then the action that I would recommend in the future.
Sometimes, I would reference evidence in the form of screenshots in the notes as well.
I mostly place reminders in there, link topics that I moved to the bin, any sanctions issued. Just as an overview. For staff I ask team leaders to leave a small note on how they did on the team if they retire for the future if they apply again.
I also have public usernotes enabled to make an entry or two public. This is useful for if the user has scammed someone in an exchange/alert people of previous bans for severe things so people are cautious if that person posts a thread in the Job Center etc.
I've never used user notes. If there's any moderation stuff I need to remember I just make a topic and list everything there. Plus that gives me a spot to fire evidence without putting into the trash and accidentally deleting it later.
Partly because when I started my forum I wanted the option to change software and in any conversions only topics really make the jump, so notes would be lost. But the major reason was just that I felt topics was easier. (Instead of having to find that member, if they changed names a lot, you could go to the topic for them and see everything listed there.)