bpo-37261: Document sys.unraisablehook corner cases (GH-14059)

bpo-37261: Document sys.unraisablehook corner cases (GH-14059)

Document reference cycle and resurrected objects issues in
sys.unraisablehook() and threading.excepthook() documentation.

Fix test.support.catch_unraisable_exception(): __exit__() no longer
ignores unraisable exceptions.

Fix test_io test_writer_close_error_on_close(): use a second
catch_unraisable_exception() to catch the BufferedWriter unraisable
exception.
(cherry picked from commit 212646cae6b7c4ddc8d98c8b9b6d39a5f259e864)

Co-authored-by: Victor Stinner <vstinner@redhat.com>