[doc] Proofreading, and improve section about FFT 2D
[doc] Rewrite FFT to be more generic

Finish the 3D case.
[src] Rewrite the FFT 2D code to match the current documentation (comments not updated though)
[src] Rewrite the FFT 3D code to match the current documentation (comments not updated though)
[src] Cosmetics, remove dead code