gh-160837: Fix asyncio call graph truncation at async generators - #160839

Add an aga_gen field to the awaitables returned by asend/athrow/aclose
on async generators, and make the graph construction inspect it.

This change originally by Timofey Ivankov (@deadlovelll), whose gh account is
frozen right now.

I made two changes:

• Dropped the doc changes added to stdtypes and documented it in inspect, where similar things are documented
• Renamed the field from ag_gen to aga_gen, since it is not actually on an async generator, but on an async generator awaitable. In the C, there are two different types and the fields are ags_gen and agt_gen, but I thought it was worth combining them.

• Issue: asyncio call graph is truncated at async generators #160837