[3.12] gh-65453: Docs - clarify AttributeError behaviour on PropertyMock (GH-121666) (GH-121969)

Fixed at EuroPython 24 sprints.
(cherry picked from commit 94e6644584d9cb08a4edcd1027e288386184816b)

Co-authored-by: Vlastimil Zíma <ziima@users.noreply.github.com>