DOC clarify attrs.define generated class example

[zihjiang]

Problem

The @attrs.define example in the “hand-written classes” section shows a __hash__ method even though the decorator's defaults produce unhashable classes. It also omits the default slots generated by attrs.define, which makes the comparison misleading.

Changes

• Add the generated __slots__ to the equivalent hand-written example.
• Remove the generated __hash__ method from the example.
• State the relevant @attrs.define defaults explicitly.

Testing

• uv run tox run -e docs-build
• uv run prek run --files docs/why.md
• git diff --check

Related issue

Closes #1472

AI disclosure

AI assistance was used to locate and draft this small documentation change. I reviewed the issue, contribution policy, resulting diff, and validation output, and I understand and take responsibility for the submitted change.