DOC: updated doc of straightening
DOC: updated doc of straightening

Former-commit-id: 13d57b1c053d39a5bb1f35f2d2f140b220700b40
Former-commit-id: afd32b4543dd5b6eb696496752cebe6d63da3224
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox