Doc update and changing x0 to S0, D
Doc update and changing x0 to S0, D

FrancoisRheaultUS/dipy,villalonreina/dipy,FrancoisRheaultUS/dipy,nilgoyyou/dipy,nilgoyyou/dipy,villalonreina/dipy,matthieudumont/dipy,matthieudumont/dipy