I suggest adding a comment help field when creating a new conditional formatting rule. Then the comment should be auto placed into cells where the rule has been activated.
The motivation behind the scenes is that each rule has its own reason of existence, then if a rule is matched the help comment will work as an assistance for users understanding how to solve an issue without having other artifacts such as legends or stuffs like that.
I am working in a spreadsheet simulating a Project Management tool like MS Project only using sheets resources I though that this new feature would be helpful.
Thanks for your attention!