Documentation improvements on sct_propseg (#1650)
Documentation improvements on sct_propseg (#1650)

* DOC: explicit parameter (fix issue #1486)

* DOC: updated sct_propseg documentation to reflect recent changes


Former-commit-id: 28973f29461ab7d14e27c8c3044981e8c4d02838
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox