Spruce up docs for flake8-pyi rules (part 2)

[AlexWaygood]

A followup PR to #10422, auditing all the docs for our PYI rules that I didn't get to in that PR.

Summary of the changes:

• Improve clarity over the motivation for some rules
• Improve links to external references. In particular, reduce links to PEPs, as PEPs are generally historical documents rather than pieces of living documentation. Where possible, it's better to link to the official typing spec, the other docs at typing.readthedocs.io/en/latest, or the docs at docs.python.org/3/library/typing.html.
• Use more concise language in a few places

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[dhruvmanila]

Reads well, thank you!