{,Range}Slider: Add release notes for valfmt arguments

Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>

Author: doronbehar

doc/users/next_whats_new/sliders_callable_valfmt.rst

@@ -0,0 +1,6 @@
+Callable *valfmt* for ``Slider`` and ``RangeSlider``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In addition to the existing %-format string, the *valfmt* parameter of
+`~.matplotlib.widgets.Slider` and `~.matplotlib.widgets.RangeSlider` now
+also accepts a callable of the form ``valfmt(val: float) -> str``.