Add in details of File_prefix which appears to have been omitted (#139)

Author: mo-rickywong

documentation/source/how_to_use_it/meshes/mesh_configuration_namelists.rst

@@ -45,6 +45,23 @@ depending on generator.
 	at z=0, as a result, features are located with only 2-coordinates,
 	``(x,y)`` in metres.
 
+  * ``file_prefix``: **'<string>'**
+      File prefix used to construct output filename(s). Depending on the logical,
+      :ref:`partition_mesh<partition_mesh>`, the output filename(s) are constructed as:
+
+      **<file_prefix>.nc**
+        Single file output for non-partitioned mesh topologies. Requested meshes
+	encompass an entire model domain.
+
+      **<file_prefix>_<partition_id>_<n_partitions>.nc**
+        Multiple files output for pre-partitioned mesh topologies. Each file
+	contains the cells of the requested meshes on a given partition of
+	the model domain. Pre-partitioned meshes allow a simple load operation
+	to populate an application's local mesh objects on a mpi rank.
+
+	**Note:** Pre-partitioned meshes are only suitable for applications
+	running with mpi ranks equal to the number of partitions.
+
   * ``geometry``: **'<string>'**
       Geometrical shape of the surface domain. Valid options: