address review: add to whatsnew

Author: skirpichev

Doc/whatsnew/3.14.rst

@@ -202,6 +202,11 @@ Other language changes
   making it a :term:`generic type`.
   (Contributed by Brian Schubert in :gh:`126012`.)
 
+* Support underscore and comma as thousands separators in the fractional part
+  for floating-point presentation types of the new-style string formatting
+  (with :func:`format` or :ref:`f-strings`).
+  (Contrubuted by Sergey B Kirpichev in :gh:`87790`.)
+
 
 New modules
 ===========