bpo-43905: Expand dataclasses.astuple() and asdict() docs (GH-26154)

Expanded ``astuple()`` docs, warning about deepcopy being applied
and providing a workaround.

Automerge-Triggered-By: GH:ericvsmith
(cherry picked from commit c1f93f0d378958dfae4f24aad0c0088e3e04e403)

Co-authored-by: andrei kulakov <andrei.avk@gmail.com>