gh-84464: Clarify how the signs of radius and extent affect turtle.circle()

[Stewyn]

The turtle.circle() docs say the arc is drawn counterclockwise whenever radius is positive. That's only true when extent is positive too: a negative extent makes the turtle move backward along the circle, which reverses the direction.

Behavior for each sign combination (standard mode, starting at the origin facing east):

call	direction	center	heading after
circle(50, 90)	counterclockwise	left	+90
circle(50, -90)	clockwise	left	−90
circle(-50, 90)	clockwise	right	−90
circle(-50, -90)	counterclockwise	right	+90

This PR updates both Doc/library/turtle.rst and the docstring to:

• say that the center is to the right of the turtle when radius is negative
• explain that the sign of extent decides forward or backward movement, so the arc is counterclockwise only when radius and extent have the same sign
• correct the final heading (the turtle turns right by extent when radius is negative)

The wording is intentionally short, following the review feedback on the earlier attempt, gh-20928, which was closed as stale.

• Issue: Documentation of turtle.circle() #84464

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2 files changed

± library/turtle.html
± whatsnew/changelog.html