[3.7] bpo-8722: Document __getattr__ behavior with AttributeError in property (GH-5543)

When `__getattr__` is implemented, attribute lookup will always fall back to that,
even if the initial failure comes from `__getattribute__` or a descriptor's `__get__`
method (including property methods).
(cherry picked from commit d1f318105b8781b01f3507d5cb0fd841b977d5f2)

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>
2 files changed