How do I add a custom comment to any folder?
If John T.'s suggestion is 'too complicated', you may use HOBComment ShellExtension:
Also available: HOBComment Explorer Extension (GUI & command line)
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You'll see your detailed pop-up description the next time you hover the mouse over that specific folder.
If you'd like to keep the normal folder icon, you can simply choose it from the selection when changing the icon, it is the exact same: