Add whatsnew entry for math module changes

Author: jepler

Doc/whatsnew/3.16.rst

@@ -93,6 +93,16 @@ gzip
   which is passed on to the constructor of the :class:`~gzip.GzipFile` class.
   (Contributed by Marin Misur in :gh:`91372`.)
 
+math
+----
+
+* Added trigonometric functions that work in units of half turns, rather than
+  radians. The new functions :func:`math.acospi`, :func:`math.asinpi`,
+  :func:`math.atanpi`, and :func:`math.atan2pi` return half-turn angles. The
+  new functions :func:`math.cospi`, :func:`math.sinpi`, and :func:`math.tanpi`
+  take half-turn angle arguments. These functions are recommended by IEEE
+  754-2019 and standardized in C23.
+
 os
 --