Found it, Setup > Attach Document.  I guess I was looking in the GL menus.  Thanks.

I entered a complicated, 19-item GL entry but forgot to add an attachment document at the time.  I would like to go back and add the document attachment now, but there seems to be no menu I can find that permits this operation.  Of course, I could void and re-enter the whole transaction again, but that would be a real pain.  It seems like a trivial step to provide a capability of later additions of a document attachment, but I am unable to discover how to do this.  Guidance?