add missing parameter in KINLsJacTimesVecFn docs

Author: gardner48

doc/kinsol/guide/source/Usage/index.rst

@@ -2157,6 +2157,15 @@ supplied, the default is a difference quotient approximation to these products.
       * ``v`` -- is the vector by which the Jacobian must be multiplied to the right.
       * ``Jv`` -- is the computed output vector.
       * ``u`` -- is the current value of the dependent variable vector.
+      * ``new_u`` -- is a flag, input from KINSOL and possibly reset by this
+        function, indicating whether the iterate vector ``u`` has been updated
+        since the last call to this function. This is useful if this function
+        computes and saves Jacobian data that depends on ``u`` for use in
+        computing :math:`J(u) v`. The input value of ``new_u`` is ``SUNTRUE``
+        following an update by KINSOL, and in that case any saved Jacobian data
+        depending on ``u`` should be recomputed. This function should then set
+        ``new_u`` to ``SUNFALSE``, so that on subsequent calls with the same
+        ``u``, the saved data can be reused.
       * ``user_data`` -- is a pointer to user data, the same as the ``user_data``
         parameter passed to :c:func:`KINSetUserData`.