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.

Start with the Sketch arc overload shown in the issue, then compare its current explanation in README.md and the CadQuery documentation at cadquery.readthedocs.io. Document what `a` and `da` represent, including direction conventions for clockwise and counterclockwise arcs; done means a reader can use this overload without guessing the parameter meanings.

Written by the indexing model from the issue text.