Update explanation for importing builtins.str (#5465)

stdlib/locale.pyi

@@ -1,4 +1,7 @@
 import sys
+
+# This module defines a function "str()", which is why "str" can't be used
+# as a type annotation or type alias.
 from builtins import str as _str
 from decimal import Decimal
 from typing import Any, Callable, Dict, Iterable, List, Mapping, Optional, Sequence, Tuple, Union