DOC: Mention --fs-subjects-dir in docs

Author: effigies

docs/outputs.rst

@@ -90,7 +90,8 @@ conformed space for surface reconstruction (``fsnative``) is stored in::
 
 FreeSurfer derivatives
 ~~~~~~~~~~~~~~~~~~~~~~
-A FreeSurfer subjects directory is created in ``<output_dir>/freesurfer``::
+A FreeSurfer subjects directory is created in ``<output dir>/freesurfer``, or the
+directory indicated with the ``--fs-subjects-dir`` flag. ::
 
     <output_dir>/
         fmriprep/

docs/workflows.rst

@@ -202,7 +202,8 @@ All surface preprocessing may be disabled with the ``--fs-no-reconall`` flag.
     Surface processing will be skipped if the outputs already exist.
 
     In order to bypass reconstruction in *fMRIPrep*, place existing reconstructed
-    subjects in ``<output dir>/freesurfer`` prior to the run.
+    subjects in ``<output dir>/freesurfer`` prior to the run, or specify an external
+    subjects directory with the ``--fs-subjects-dir`` flag.
     *fMRIPrep* will perform any missing ``recon-all`` steps, but will not perform
     any steps whose outputs already exist.