DOC: crop_image: clarified comments
DOC: crop_image: clarified comments

Former-commit-id: 6291baccd174b7187a7c66051c50ab049539fa20
Former-commit-id: 358df2eba33e9813258e14cba84c229d96d22540
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox