[3.12] gh-105857: Document that asyncio subprocess std{in,out,err} can be file handles (GH-107986) (#108332)

(cherry picked from commit 13966da71b693b1fae1a8ef66e34e2f0a90ec6c0)

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