Adapted a mail from Mark about some design and add it as documentation for the FFT module.
Adapted a mail from Mark about some design and add it as documentation for
the FFT module.

Zefz/eigen,ritsu1228/eigen,toastedcrumpets/eigen,pasuka/eigen,pasuka/eigen,pasuka/eigen,TSC21/Eigen,Zefz/eigen,ritsu1228/eigen,toastedcrumpets/eigen,ROCmSoftwarePlatform/hipeigen,Zefz/eigen,toastedcrumpets/eigen,pasuka/eigen,ROCmSoftwarePlatform/hipeigen,TSC21/Eigen,ROCmSoftwarePlatform/hipeigen,ritsu1228/eigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,pasuka/eigen,TSC21/Eigen,ritsu1228/eigen,Zefz/eigen,TSC21/Eigen,toastedcrumpets/eigen