[3.12] gh-88531 Fix dataclass __post_init__/__init__ interplay documentation (gh-107404) (#114162)

* Simplify __post_init__ example usage. It applies to all base classes, not just dataclasses.
(cherry picked from commit 05008c27b73da640b63c0d335c65ade517c0eb84)

Co-authored-by: Steffen Zeile <48187781+Kaniee@users.noreply.github.com>