EAMxx: Add doc strings for SL transport.

Author: ambrad

components/eamxx/cime_config/namelist_defaults_eamxx.xml

@@ -895,10 +895,10 @@ be lost if SCREAM_HACK_XML is not enabled.
     <!-- SL transport settings. SL defaults to on for pg2 configs. -->
     <dt_tracer_factor constraints="ge 1" hgrid=".*pg2">6</dt_tracer_factor>
     <hypervis_subcycle_q hgrid=".*pg2">6</hypervis_subcycle_q>
-    <transport_alg hgrid=".*pg2">12</transport_alg>
-    <semi_lagrange_trajectory_nsubstep>0</semi_lagrange_trajectory_nsubstep>
-    <semi_lagrange_trajectory_nvelocity>-1</semi_lagrange_trajectory_nvelocity>
-    <semi_lagrange_halo>-1</semi_lagrange_halo>
+    <transport_alg hgrid=".*pg2" doc="0 (Eulerian) or 12 (semi-Lagrangian) tracer transport">12</transport_alg>
+    <semi_lagrange_trajectory_nsubstep doc="Controls trajectory method. 0: original method. 1: New method. 2 or more: New method with this many trajectory substeps per tracer time step.">0</semi_lagrange_trajectory_nsubstep>
+    <semi_lagrange_trajectory_nvelocity doc="Number of velocity slices to use in new method. 2 or less maps to 2">-1</semi_lagrange_trajectory_nvelocity>
+    <semi_lagrange_halo doc="Max number of element halos available in communication. -1 triggers an automatic estimate.">-1</semi_lagrange_halo>
     <semi_lagrange_diagnostics>0</semi_lagrange_diagnostics>
     <!-- Other settings that we'll trigger based on pg2 for convenience -->
     <se_ftype valid_values="0,2" hgrid=".*pg2">2</se_ftype>