fix docstring for add_particles

bfps/NavierStokes.py

@@ -423,7 +423,8 @@ class NavierStokes(bfps.fluid_base.fluid_particle_base):
         :type acc_name: str
 
         .. warning:: if not None, kcut must be a list of decreasing
-        wavenumbers, since filtering is done on the same field...
+                     wavenumbers, since filtering is done sequentially
+                     on the same complex FFTW field.
         """
         if self.dtype == np.float32:
             FFTW = 'fftwf'