op/clip: document dimensions of Border.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

op/clip/shapes.go

@@ -47,17 +47,20 @@ func (rr RRect) Add(ops *op.Ops) {
 	rr.Op(ops).Add(ops)
 }
 
-// Border represents the clip area of a rectangular border.
+// Border represents a rectangular border.
 type Border struct {
 	// Rect is the bounds of the border.
-	Rect   f32.Rectangle
+	Rect f32.Rectangle
+	// Width of the line tracing Rect.
 	Width  float32
 	Dashes DashSpec
 	// The corner radii.
 	SE, SW, NW, NE float32
 }
 
-// Op returns the Op for the border.
+// Op returns the clip operation for the border. Its area corresponds to a
+// stroked line that traces the border rectangle, optionally with rounded
+// corners and dashes.
 func (b Border) Op(ops *op.Ops) Op {
 	var p Path
 	p.Begin(ops)