Improve documentation on longitudinal option.

[bpinsard]

Please describe how the documentation should be improved.
I recently ran smriprep with the longitudinal option, and I thought it would run freesurfer longitudinal full pipeline until I got the results.
Now I understand that it's only in the robust template creation that it makes a difference.
I think it would be great to expand a bit the documentation on that option to mention what it changes in the workflow.

I also wonder if a precomputed longitudinal freesurfer could be used as input for smriprep/fmriprep?

[effigies]

Maybe we should link to https://fmriprep.org/en/stable/workflows.html#longitudinal-processing in the CLI outputs? I'm not sure what else to say.

And #21 is a long-standing issue. If someone with experience working with longitudinal FreeSurfer data is willing to contribute the necessary patches, then we can support it. I don't know enough to know what that would look like.