Document sys.std* typing (#10135)

stdlib/sys.pyi

@@ -57,9 +57,20 @@ if sys.version_info >= (3, 8):
     pycache_prefix: str | None
 ps1: object
 ps2: object
+
+# TextIO is used instead of more specific types for the standard streams,
+# since they are often monkeypatched at runtime. At startup, the objects
+# are initialized to instances of TextIOWrapper.
+#
+# To use methods from TextIOWrapper, use an isinstance check to ensure that
+# the streams have not been overridden:
+#
+# if isinstance(sys.stdout, io.TextIOWrapper):
+#    sys.stdout.reconfigure(...)
 stdin: TextIO
 stdout: TextIO
 stderr: TextIO
+
 if sys.version_info >= (3, 10):
     stdlib_module_names: frozenset[str]