DOC add missing bugfix to what's new

Author: jnothman

doc/whats_new.rst

@@ -212,6 +212,10 @@ Bug fixes
      left `coef_` as a list, rather than an ndarray.
      :issue:`8160` by :user:`CJ Carey <perimosocordiae>`.
 
+   - Fix a bug in cases where `numpy.cumsum` may be numerically unstable,
+     raising an exception if instability is identified.  :issue:`7376` and
+     :issue:`7331` by `Joel Nothman`_ and :user:`yangarbiter`.
+
 API changes summary
 -------------------