DOC: improved usage() and added todo
DOC: improved usage() and added todo

Former-commit-id: f13a1388e3f791427a57cf265e6fe6e6046bec2d [formerly 7f00fa0ce46294cc6aaf6fee8c87e52b7fe50cb3]
Former-commit-id: 883977f7b5574a9d48637e2a99b0e4d8dc5a0b2f
Former-commit-id: 6af679272cb83745781122daedb8806db0363e21
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox