pythongh-86726: Note that the ttk Treeview separator column option needs Tk 9.0 (pythonGH-153556)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/tkinter.ttk.rst

@@ -884,8 +884,8 @@ This widget accepts the following specific options:
    The *selectmode* option gained the values ``"single"`` and ``"multiple"``;
    the new widget options *selecttype* (``"item"`` or ``"cell"`` selection),
    *striped* (zebra-striped rows), and *titlecolumns* / *titleitems* (columns
-   or rows frozen against scrolling) were introduced; and items gained a
-   *hidden* option.
+   or rows frozen against scrolling) were introduced; the column *separator*
+   option was added; and items gained a *hidden* option.
    Tk 9.1 added the *rowheight* and *headingheight* options.