sct_label_utils: added docstrings to function remove_or_keep_labels
sct_label_utils: added docstrings to function remove_or_keep_labels


Former-commit-id: 51f1c09f449fafd9ca7b87e84c24677c31c61bd3
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox