Follow-up: quasi-3D IGF solver (#5516)

This is a minor follow-up to PR #5089.
It fixes two comments and adds the documentation entry about
`ablastr.nprocs_igf_fft`.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Docs/source/usage/parameters.rst

@@ -290,14 +290,21 @@ Overall simulation parameters
         In electromagnetic mode, this solver can be used to initialize the species' self fields
         (``<species_name>.initialize_self_fields=1``) provided that the field BCs are PML (``boundary.field_lo,hi = PML``).
 
-          * ``warpx.use_2d_slices_fft_solver`` (`bool`, default: 0): Select the type of Integrated Green Function solver.
+          * ``warpx.use_2d_slices_fft_solver`` (`bool`) optional (default: 0): Select the type of Integrated Green Function solver.
             If 0, solve Poisson equation in full 3D geometry.
             If 1, solve Poisson equation in a quasi 3D geometry, neglecting the :math:`z` derivatives in the Laplacian of the Poisson equation.
             In practice, in this case, the code performes many 2D Poisson solves on all :math:`(x,y)` slices, each slice at a given :math:`z`.
             This is often a good approximation for ultra-relativistic beams propagating along the :math:`z` direction, with the relativistic solver.
             As a consequence, this solver does not need to do an FFT along the :math:`z` direction,
             and instead uses only transverse FFTs (along :math:`x` and :math:`y`) at each :math:`z` position (or :math:`z` "slice").
 
+          * ``ablastr.nprocs_igf_fft`` (`int`) optional (default: number of MPI ranks): Number of MPI ranks used to parallalelize the FFT solver.
+            This can be less or equal than then number of MPI ranks that are used to run the overall simulation.
+            It can be useful if the auxiliary simulation boxes fit within a single process, so to avoid extra communications.
+            The auxiliary boxes are extended boxes in real and spectral space that are used to perform the necessary FFTs.
+            The extended simulation box size in real space is :math:`2n_x-1, 2n_y-1, 2n_z-1` with the 3D solver, :math:`2n_x-1, 2n_y -1, n_z` with the 2D solver.
+            The extended simulation box size in spectral space is :math:`n_x, 2n_y-1, 2n_z-1` with the 3D solver, :math:`n_x, 2n_y-1, n_z` with the 2D solver.
+
 * ``warpx.self_fields_required_precision`` (`float`, default: 1.e-11)
     The relative precision with which the electrostatic space-charge fields should
     be calculated. More specifically, the space-charge fields are

Source/ablastr/fields/IntegratedGreenFunctionSolver.cpp

@@ -70,7 +70,7 @@ computePhiIGF ( amrex::MultiFab const & rho,
     amrex::Real const dz = cell_size[2];
 
     if (!is_igf_2d_slices){
-        // 2D sliced solver
+        // fully 3D solver
         obc_solver->setGreensFunction(
         [=] AMREX_GPU_DEVICE (int i, int j, int k) -> amrex::Real
         {
@@ -84,7 +84,7 @@ computePhiIGF ( amrex::MultiFab const & rho,
             return SumOfIntegratedPotential3D(x, y, z, dx, dy, dz);
         });
     }else{
-        // fully 3D solver
+        // 2D sliced solver
         obc_solver->setGreensFunction(
         [=] AMREX_GPU_DEVICE (int i, int j, int k) -> amrex::Real
         {