[3.12] gh-105857: Document that asyncio subprocess std{in,out,err} can be file handles (GH-107986)

[miss-islington]

stdin/out can be filehandles -> add to docs.
(cherry picked from commit 13966da)

Co-authored-by: Hadházy Tamás 85063808+Hels15@users.noreply.github.com

• Issue: asyncio subprocess stdin/out/err can be filehandles, but this is undocumented #105857

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