[3.12] gh-120773: document introspective attributes of an async generator object in the inspect module (GH-120778) (#120828)

gh-120773: document introspective attributes of an async generator object in the inspect module (GH-120778)
(cherry picked from commit 83d3d7aace32b8536f552f78dd29610344f13160)

Co-authored-by: blhsing <blhsing@gmail.com>