[docs only] remove comment on derivatives, mentioning the old AFNI skullstripping
[docs only] remove comment on derivatives, mentioning the old AFNI skullstripping

oesteban/preprocessing-workflow,oesteban/fmriprep,poldracklab/fmriprep,poldracklab/fmriprep,oesteban/fmriprep,poldracklab/preprocessing-workflow,poldracklab/preprocessing-workflow,oesteban/fmriprep,poldracklab/fmriprep,oesteban/preprocessing-workflow