Hi Andrew,
That's right, slurs are quadratic beziers and you can adjust the control points. If you have used Adobe Illustrator it should be clear, for the bezier curves there are the same.
Possibly more than you need to know, but some nice illustrations:
There's a much better slur shaping function called /shapeII available in openlilylib. It has some small shortcomings or limitations but it is easier to use and tweak. It has some good ways to specify the control points that are not absolute coordinates. I think sometimes my scores consist entirely of calls to /shapeII, so heavily do I use it. It works reliably for me.