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.

從 issue 中展示的 Sketch 圓弧多載開始，然後將其在 README.md 中的目前說明與 cadquery.readthedocs.io 上的 CadQuery 文件進行比較。記錄 `a` 和 `da` 分別代表什麼，包括順時針和逆時針圓弧的方向慣例；當讀者無需猜測參數含義即可使用此多載時，即視為完成。

由索引模型根據 Issue 內容生成。