@Rellwood - IIRC you use Windows.
If that's so, you could add a desktop.ini file to all your library folders, and put a description into the Infotip element, when you hover on the library folder in File explorer you'll see your description, thus:
You'll need to tell calibre's Check library function to ignore the desktop.ini, viz:
Another thought (probably a better one for you): Use the Notebook feature of Comfy's Reminder plugin to store a list of library names and descriptions.
BR