Summary

Clarify that the @dataclass decorator is not automatically applied to
subclasses. Undecorated subclasses inherit dataclass fields and generated
methods through the normal MRO, so fields from multiple dataclass bases are not
combined unless the subclass is itself decorated.

Also clarify that kw_only=True applies only to fields defined by the class
being decorated and does not change inherited fields.

The existing reverse-MRO description already covers decorated subclasses, and
the inherited default-ordering error remains documented with the decorator, so
this change avoids duplicating those rules in the inheritance section.

I used Codex to help investigate the behavior and draft the wording. I reviewed and validated the change myself.

Closes #139497.

Validation

• make -C Doc check
• make -C Doc html (Sphinx --fail-on-warning)
• Focused runtime proof on current main covering decorated and undecorated
  multiple inheritance, redecorated subclasses, and inherited fields under
  kw_only=True