add fix of GH14922 to release notes for 0.20.0

Author: uweschmitt

doc/source/whatsnew/v0.20.0.txt

@@ -246,6 +246,7 @@ Bug Fixes
 - Bug in ``DataFrame`` construction in which unsigned 64-bit integer elements were being converted to objects (:issue:`14881`)
 - Bug in ``astype()`` where ``inf`` values were incorrectly converted to integers. Now raises error now with ``astype()`` for Series and DataFrames (:issue:`14265`)
 - Bug in ``describe()`` when passing a numpy array which does not contain the median to the ``percentiles`` keyword argument (:issue:`14908`)
+- Bug in ``sort_values()`` when sorting by multiple columns where one colums is of type `int64` and contains `NaT` (:issue:`14922`)