Issue python#24861: Add docstring to idlelib.__init__ with 'private' warning.

Author: terryjreedy

Lib/idlelib/__init__.py

@@ -1 +1,8 @@
-# Dummy file to make this a package.
+"""The idlelib package implements the Idle application.
+
+Idle includes an interactive shell and editor.
+Use the files named idle.* to start Idle.
+
+The other files are private implementations.  Their details are subject to
+change.  See PEP 434 for more.  Import them at your own risk.
+"""