bpo-42560: add warning to Tkinter docs about outdated pre-8.5 documentation online (GH-27836)

Co-authored-by: Łukasz Langa <lukasz@langa.pl> Co-authored-by: E-Paine <63801254+E-Paine@users.noreply.github.com> (cherry picked from commit 1eb4510) Co-authored-by: Mark Roseman <mark@markroseman.com>

Author: miss-islington

Doc/library/tkinter.rst

@@ -29,6 +29,13 @@ make the experience more pythonic. This documentation will concentrate on these
 additions and changes, and refer to the official Tcl/Tk documentation for
 details that are unchanged.
 
+.. note::
+
+ Tcl/Tk 8.5 (2007) introduced a modern set of themed user interface components
+ along with a new API to use them. Both old and new APIs are still available.
+ Most documentation you will find online still uses the old API and
+ can be woefully outdated.
+
 .. seealso::
 
  * `TkDocs <http://tkdocs.com/>`_