gh-108973: Fix asyncio test_subprocess_consistent_callbacks() (#109431)

SubprocessProtocol process_exited() method can be called before
pipe_data_received() and pipe_connection_lost() methods. Document it
and adapt the test for that.

Revert commit 282edd7b2a74c4dfe1bfe3c5b1d30f9c21d554d6.
_child_watcher_callback() calls immediately _process_exited(): don't
add an additional delay with call_soon(). The reverted change didn't
make _process_exited() more determistic: it can still be called
before pipe_connection_lost() for example.

Co-authored-by: Davide Rizzo <sorcio@gmail.com>