If you want to delete all files that are on the USB that are not on the PC then I believe you can do this:
# deletes any files that are on the USB that are not on the PC and synchronizes any files that have been changed more recently on the PC then on the USB.
rsync -auv --delete ~/Documents/ /media/20CC-A5E5/Documents/
# synchronizes files that are newer on the USB then on the PC
rsync -auv /media/20CC-A5E5/Documents/ ~/Documents/
By having the first command use -u, you should not overwrite any files that are on the USB and the PC, and by using --delete you will delete all files that are on the USB that are not on the PC.
The second command then synchronizes the USB with the PC.
The one downside to doing this is that ANY files that are on the USB and are not on the PC will be deleted. This is what you want I think though. If you create any new files on the USB that are not on the PC the new files on the USB will be deleted. If you delete any files on the PC they will be deleted on the USB.
From what you want I don't think this will suffice, but it is the most you can do with RSync.
If you wanted to just delete files on the USB that were DELETED on the PC, and NOT delete files on the USB that were never on the PC, then you would have to create some sort of program that would log every time you deleted a file on your PC, and then only delete those from the USB. Which is probably a lot more complicated and more work than you want to put in for this.