update doc for save potential

Author: elindgren

doc/nep/input_parameters/save_potential.rst

@@ -9,10 +9,11 @@ This keyword sets the number of of generations between writing a ``nep.txt`` che
 If ``<format>`` is set to ``0`` the output file name is formatted as ``nep_gen[generation].txt``.
 If ``<format>`` is set to ``1`` the output file name is formatted as ``nep_y[year]_m[month]_d[day]_h[hour]_m[minute]_s[second]_generation[generation].txt``.
 These model files can be used to monitor the training progress of your model.
+Additionally, if ``<save_restart>`` is set to ``1`` the :ref:`nep.restart file <nep_restart>` is also written, following the naming format set by ``<format>``.
 Note that the :ref:`nep.restart file <nep_restart>` is the file that is required to continue training.
 
 The syntax is::
 
-  save_potential <number_of_generations_between_save_potential> <format>
+  save_potential <number_of_generations_between_save_potential> <format> <save_restart>
 
 The default number of generations between saved model files is :math:`N=10^5` and the output file names use the extended format.