[3.13] pythongh-120773: document introspective attributes of an async…

… generator object in the inspect module (pythonGH-120778) (python#120827)

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

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