DOC: updated docstrings of streamtube and line according to Ariel's suggestions
DOC: updated docstrings of streamtube and line according to Ariel's suggestions

StongeEtienne/dipy,nilgoyyou/dipy,JohnGriffiths/dipy,villalonreina/dipy,villalonreina/dipy,FrancoisRheaultUS/dipy,JohnGriffiths/dipy,nilgoyyou/dipy,matthieudumont/dipy,FrancoisRheaultUS/dipy,matthieudumont/dipy,StongeEtienne/dipy