Correct the unit of diagnostic "eta_cor"

It should be in thickness unit (m for Boussinesq and kg/m2 for
non-Boussinesq). The long name description is also slightly changed.

Author: herrwang0

src/core/MOM_barotropic.F90

@@ -6315,7 +6315,7 @@ subroutine barotropic_init(u, v, h, Time, G, GV, US, param_file, diag, CS, &
   CS%id_vbtav = register_diag_field('ocean_model', 'vbtav', diag%axesCv1, Time, &
       'Barotropic time-average meridional velocity', 'm s-1', conversion=US%L_T_to_m_s)
   CS%id_eta_cor = register_diag_field('ocean_model', 'eta_cor', diag%axesT1, Time, &
-      'Corrective mass flux within a timestep', 'm', conversion=GV%H_to_m)
+      'Corrective mass or volume flux within a timestep', thickness_units, conversion=GV%H_to_MKS)
   CS%id_visc_rem_u = register_diag_field('ocean_model', 'visc_rem_u', diag%axesCuL, Time, &
       'Viscous remnant at u', 'nondim')
   CS%id_visc_rem_v = register_diag_field('ocean_model', 'visc_rem_v', diag%axesCvL, Time, &