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.
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 55686d7..fc474c9 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -2072,8 +2072,12 @@
         writer.close = lambda: None
         writer = None
 
+        # Ignore BufferedWriter (of the BufferedRWPair) unraisable exception
         with support.catch_unraisable_exception():
-            pair = None
+            # Ignore BufferedRWPair unraisable exception
+            with support.catch_unraisable_exception():
+                pair = None
+                support.gc_collect()
             support.gc_collect()
 
     def test_reader_writer_close_error_on_close(self):