gh-157847: Improve more turtle docstrings and docs

[StanFromIreland]

This continues the work started in #158281 and #158330. It improves the reference docs and docstrings, modernising them, making them easier to understand/clearer, and consistent.

Yes, a few could reasonably be classified as churn, however, the turtle module is, frankly, quite messy in places. I think these changes improve readability and, as a result, maintainability, so I intend to tidy up similar issues. They should also have relatively little impact on useful Git history, since the majority of the affected lines date back to the original port in 97cf99f.

• Issue: Restructure the turtle documentation #157847

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #34886064 | 📁 Comparing f3f5085 against main (0906d2a)

  🔍 Preview build  

2 files changed

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

[terryjreedy]

To review, I needed to experiment. To refresh my memory, I followed the tutorial. In "Starting a turtle environment", the import is insufficient. It does not 'start a turtle environment'. Add reset() to the code example and text "The reset command opens a default screen with a default turtle at its default position and orientation." directly below. Starting with forward 'worked' in that it first calls reset, but it was jarring and uncomfortable doing so when there was nothing visible.

I would add this to this PR.

[StanFromIreland]

Yeah, the tutorial (like all of the sections) needs some work. But, I'm happy to apply your suggestion now if that aids reviewing :-) However, I switched reset() for showturtle().

[bedevere-app]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[terryjreedy]

Yes, showturtle() is a better name with the same visible effect.

[StanFromIreland]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@terryjreedy: please review the changes made to this pull request.