Add doxygen documentation regarding is_igf_2d_slices

[aeriforme]

Add Doxygen documentation regarding is_igf_2d_slices.
This is a boolean flag that lets the user select between two types of FFT-based Poisson solver:

• fully 3D: solves Poisson equation in 3D geometry
• quasi-3D: solves many 2D Poisson equations, one for each z slice, independently one another

[ax3l]

Thank you, Arianna! :)

[ax3l]

@EZoni if we want to catch this and get WarpX up to par with ImpactX, we need to finish #3560 :)