Clarify FoF select-output docs

doc/RTD/source/FriendsOfFriends/stand_alone_fof.rst

@@ -35,6 +35,9 @@ additional field for each particle. This contains the ``GroupID`` of
 each particle and can be used to find all the particles in a given
 halo and to link them to the information stored in the catalogue.
 
+The particle fields written to the snapshot can be modified using the 
+:ref:`Output_selection_label` options.
+
 .. warning::
 
    Note that since not all particle properties are read in stand-alone

doc/RTD/source/ParameterFiles/output_selection.rst

@@ -74,6 +74,9 @@ CGS. Entries in the file look like:
     SmoothingLengths_Gas: on  # Co-moving smoothing lengths (FWHM of the kernel) of the particles : a U_L  [ cm ]
     ...
 
+This can also be used to set the outputs produced by the 
+:ref:`fof_stand_alone_label`.
+
 For cosmological simulations, users can optionally add the ``--cosmology`` flag
 to generate the field names appropriate for such a run.