This sounds to me like the documentation could be improved. Here’s a start though:
The DrawArc method draws an arc within a given rectangle. Specifically, imagine that it draws an oval (or circle) within the rectangle defined by the left, top, bottom, and right coordinates. The startAngle indicates at what angle (in degrees clockwise from the +X axis) to start drawing the arc. The sweepAngle indicates the width of the arc in degrees, starting from the startAngle. The useCenter property controls whether the arc connects to the center (like a pie chart) or not (in which case a chord is drawn from the start to end of the arc). Lastly, the fill parameter determines whether the arc should be filled in with the Canvas’s PaintColor or not.
Note that the top left of the canvas is 0, 0 and the X and Y coordinates grow to the right and down, respectively.