Mention multiple track files when run with MPI in documentation

docs/source/usersguide/scripts.rst

@@ -112,7 +112,7 @@ otherwise.
 tallies. The path to the statepoint file can be provided as an optional arugment
 (if omitted, a file dialog will be presented).
 
-.. _scripts_track:
+.. _scripts_track_combine:
 
 ------------------------
 ``openmc-track-combine``
@@ -125,6 +125,8 @@ filename can also be changed with the ``-o`` flag:
 
 -o OUT, --out OUT    Output HDF5 particle track file
 
+.. _scripts_track:
+
 -----------------------
 ``openmc-track-to-vtk``
 -----------------------

docs/source/usersguide/settings.rst

@@ -589,3 +589,13 @@ The full list of fields is as follows:
   :wgt: Weight
   :cell_id: Cell ID
   :material_id: Material ID
+
+.. note:: If you are using an MPI-enabled install of OpenMC and run a simulation
+          with more than one process, a separate track file will be written for
+          each MPI process with the filename ``tracks_p#.h5`` where # is the
+          rank of the corresponding process. Multiple track files can be
+          combined with the :ref:`scripts_track_combine` script:
+
+          .. code-block:: sh
+
+            openmc-track-combine tracks_p*.h5 --out tracks.h5