Doc: Clarify doc about turtle.reset() and angle measurement unit

Documentation

The turtle module has a couple of functions that could cause misconceptions about the reset function and angle measurement unit.

turtle.reset():

Delete the turtle’s drawings from the screen, re-center the turtle and set variables to the default values.

1. Re-center is not always true. It depends on the turtle mode. I think the right thing to say is something like "send the turtle to the (0, 0) position".
2. What variables?

turtle.left()/rigth():

Turn turtle left/rigth by angle units. (Units are by default degrees, but can be set via the [degrees()](https://docs.python.org/3/library/turtle.html#turtle.degrees) and [radians()](https://docs.python.org/3/library/turtle.html#turtle.radians) functions.) Angle orientation depends on the turtle mode, see [mode()](https://docs.python.org/3/library/turtle.html#turtle.mode).

It is said that the unit defaults are degrees. But if we run turtle.radians() and after turtle.reset(), the unit doesn't change to degrees.

turtle.degrees(fullcircle=360.0)

Set angle measurement units, i.e. set number of “degrees” for a full circle. Default value is 360 degrees.

turtle.radians()

Set the angle measurement units to radians. Equivalent to degrees(2*math.pi).

These two last functions say nothing about the default unit. (and missing a "See also" in each)

Linked PRs

• gh-138879
• gh-138882

从 turtle 模块文档开始，检查 reset()、left()/right()、degrees()、radians() 和 mode() 的条目。明确 reset() 的行为和角度单位的默认值，说明哪些变量会被重置，并添加缺失的 See also 引用；链接的 PRs 表明这项工作可能已经在进行中。