bpo-42560: reorganize Tkinter docs modules section for clarity (GH-27840)

 bpo-42560: reorganize Tkinter docs modules section for clarity (GH-27840)

- move description of internal modules (_tkinter and tkinter.constants) from section intro to list of additional modules at end of section, as not most important info
- added missing ttk and tix here
- emphasized up front that most apps will need tkinter and ttk