[3.13] gh-120773: document introspective attributes of an async generator object in the inspect module (GH-120778) (#120827)

gh-120773: document introspective attributes of an async generator object in the inspect module (GH-120778)
(cherry picked from commit 83d3d7a)

Co-authored-by: blhsing <[email protected]>

Author: miss-islington