[3.11] gh-108973: Fix asyncio SubprocessProtocol doc (#109431) (#109610)

gh-108973: Fix asyncio SubprocessProtocol doc (#109431)

SubprocessProtocol process_exited() method can be called before
pipe_data_received() and pipe_connection_lost() methods. Document it
and adapt the example in the doc.

Co-authored-by: Davide Rizzo <sorcio@gmail.com>
(cherry picked from commit ced6924630037f1e5b3d1dbef2b600152fb07fbb)