Update documentation for inactive top cell test

Author: cbegeman

docs/developers_guide/ocean/test_groups/global_ocean.rst

@@ -704,6 +704,11 @@ If a baseline is provided, prognostic variables as well as ecosystem tracers
 included in the mesh) are compared with a baseline, and the
 ``time integration`` timer is compared with that of the baseline.
 
+This test case can also serve as a verification of the inactive top cell
+implementation. If ``inactive_top_cells`` is given in the config file with 
+values greater than zero, the vertical domain is shifted downward by the given 
+number of layers.
+
 .. _dev_ocean_global_ocean_restart_test:
 
 restart_test test case