Clarify slab decomposition and optional arguments

docs/source/pages/api_domain.rst

@@ -156,17 +156,26 @@ The algorithm will choose the closest combination such as
   n\_proc=n\_col=\sqrt{nproc}
 
 In case the root is not exact the closest combitation to have :math:`n\_proc \approx n\_col` with 
-`n\_proc <  n\_col` is used. 
+`n\_proc <  n\_col` is used.
+If a 1D slab decomposition is needed instead of a 2D pencil one, it is recommended to set ``p_row`` to unity and ``p_col`` to ``nproc``.
 
 An optional parameter may be passed to this initialisation routine:
 
 ::
 
- call decomp_2d_init(nx, ny, nz, p_row, p_col,periodic_bc)
+ call decomp_2d_init(nx, ny, nz, p_row, p_col, periodic_bc)
 
-Here periodic_bc is a 1D array containing 3 logical values that specify whether periodic boundary condition 
+Here ``periodic_bc`` is a 1D array containing 3 logical values that specify whether periodic boundary condition 
 should apply in certain dimensions. Note this is only applicable if halo-cell communication is to be used.
 
+Another optional parameter may be passed at the initialization stage:
+
+::
+
+ call decomp_2d_init(nx, ny, nz, p_row, p_col, periodic_bc, comm)
+
+Here ``comm`` is the MPI communicator that the library will use. By default, MPI_COMM_WORLD is used.
+
 A key element of this library is a set of communication routines that actually perform the data transpositions. 
 As mentioned, one needs to perform 4 global transpositions to go through all 3 pencil orientations. 
 Correspondingly, the library provides 4 communication subroutines: