[`pydocstyle`] Clarify which quote styles are allowed (`D300`) (#22825)
## Summary
This PR updates the documentation for the `D300`
(`triple-single-quotes`) rule.
The description now explicitly mentions that the rule flags both single
quotes (`'`) and triple single quotes (`'''`). I've also added a Python
example to show the single-quote violation.
Documentation was regenerated using `cargo run -p ruff_dev --
generate-docs`.
## Fixes
Fixes #20838