There is no sufficient document for Sketch's arc function.

In the current master, there is no sufficient document for Sketch's arc function, especially for the overload function of
```
@arc.register
def arc(
self: T,
c: Point,
r: Real,
a: Real,
da: Real,
tag: Optional[str] = None,
forConstruction: bool = False,
) -> T:
```
Maybe variable "a" and variable "da" should be given more cares.

From the source code, I guess "a" should be the start angle, and "da" should be the angle value with direction included. If the arc is toward CW, the "da" should be negtive. Otherwise, the "da" should be positive.