gh-152587: Stop documenting bogus default values in tkinter variable methods (GH-152594)

The name parameter of Misc.wait_variable(), setvar() and getvar() and the
value parameter of setvar() should not be optional.  Their default values
('PY_VAR' and '1') are not meaningful and should not be advertised.

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

Author: serhiy-storchaka

Doc/library/tkinter.rst

@@ -1056,11 +1056,11 @@ Base and mixin classes
       :class:`int`.
       Raise :exc:`ValueError` if *s* is not a valid integer.
 
-   .. method:: getvar(name='PY_VAR')
+   .. method:: getvar(name)
 
       Return the value of the Tcl global variable named *name*.
 
-   .. method:: setvar(name='PY_VAR', value='1')
+   .. method:: setvar(name, value)
 
       Set the Tcl global variable named *name* to *value*.
 
@@ -1524,10 +1524,10 @@ Base and mixin classes
       This updates the display of windows, for example after geometry changes,
       but does not process events caused by the user.
 
-   .. method:: waitvar(name='PY_VAR')
+   .. method:: waitvar(name)
       :no-typesetting:
 
-   .. method:: wait_variable(name='PY_VAR')
+   .. method:: wait_variable(name)
 
       Wait until the Tcl variable *name* is modified, continuing to process
       events in the meantime so that the application stays responsive.