I find this paragraph to be hard to imagine without a diagram:
"The arc is constructed such that the current position of the sketch is placed along an imaginary circle of the specified radius, at angleStart degrees. The resulting arc is the segment of the imaginary circle from that origin point to angleEnd, radius away from the center of the imaginary circle."
Also, I note that KCL book doesn't touch on arc() at all. An in-depth description in one of the two (or both) places would help.