IDLE: Add util and stub example comments (python#113222)

Author: terryjreedy

Lib/idlelib/idle_test/example_stub.pyi

@@ -1,2 +1,4 @@
+" Example to test recognition of .pyi file as Python source code.
+
 class Example:
     def method(self, argument1: str, argument2: list[int]) -> None: ...

Lib/idlelib/util.py

@@ -13,9 +13,9 @@
     * warning stuff (pyshell, run).
 """
 
-# .pyw is for Windows; .pyi is for stub files.
-py_extensions = ('.py', '.pyw', '.pyi')  # Order needed for open/save dialogs.
-
+# .pyw is for Windows; .pyi is for typing stub files.
+# The extension order is needed for iomenu open/save dialogs.
+py_extensions = ('.py', '.pyw', '.pyi')
 
 if __name__ == '__main__':
     from unittest import main