[3.9] gh-92436: __future__ docs: add note on expectations for "from __future__...

[3.9] gh-92436: __future__ docs: add note on expectations for "from __future__ import annotations" (GH-92568). (#92726)

(cherry picked from commit 6582c964)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>