I bought an original illustration about ten years ago and the artist included a document like what you're describing, Marsha. He explained it to me before I made the purchase so it wasn't a surprise to receive that with the artwork.
I was new to buying art and initially I felt like I should own the rights since I was buying the piece but I did some research and learned (of course) that what the artist was doing was normal. So I had resolved my own understanding of it before we completed the transaction.
I think it was wise of him, and I think it's wise of you to spell it out to your friends and customers.