add 3d distributed fft flag to docs

Docs/source/usage/parameters.rst

@@ -265,9 +265,15 @@ 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): if 0, solve Poisson equation in full 3D geometry with the
- Integrated Green Function solver; if 1, solve Poisson equation in a quasi 3D geometry. In the latter case,
- the code performes many 2D Poisson solves for each slice at a given :math:`z`.
+ * warpx.use_2d_slices_fft_solver (`bool`, 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. In this case, the code performes many 2D Poisson solves 
+ on all :math:`(x,y)` slices, each slice at a given :math:`z`.
+
+ * warpx.use_distributed_3d_fft_solver (`bool`, default: 0): Choose whether the 3D FFTs performed in the 
+ full 3D Integrated Green Function solver are distributed. 
+ If 0, the FFTs are performed on a single process (the rest of the code is still fully parallel). 
+ If 1, the FFTs are distributed using the heFFTe library. The code must be compiled with `-DWarpX_HEFFTE=ON`. 
 
 * ``warpx.self_fields_required_precision`` (`float`, default: 1.e-11)
  The relative precision with which the electrostatic space-charge fields should