Add documentation for the fields

Source/Fields.H

@@ -19,23 +19,23 @@ namespace warpx::fields
 {
     AMREX_ENUM(FieldType,
         None,
-        Efield_aux,
-        Bfield_aux,
-        Efield_fp,
-        Bfield_fp,
-        Efield_fp_external,
-        Bfield_fp_external,
-        current_fp,
-        current_fp_nodal,
-        current_fp_vay,
-        current_buf,
-        current_store,
-        rho_buf,
-        rho_fp,
-        F_fp,
-        G_fp,
-        phi_fp,
-        vector_potential_fp,
+        Efield_aux, // Field that the particles gather from. Obtained from Efield_fp (and Efield_cp when using MR); see UpdateAuxilaryData
+        Bfield_aux, // Field that the particles gather from. Obtained from Bfield_fp (and Bfield_cp when using MR); see UpdateAuxilaryData
+        Efield_fp,  // The field that is updated by the field solver at each timestep
+        Bfield_fp,  // The field that is updated by the field solver at each timestep
+        Efield_fp_external, // Stores grid particle fields provided by the user as  through an openPMD file
+        Bfield_fp_external, // Stores grid particle fields provided by the user as  through an openPMD file
+        current_fp, // The current that is used as a source for the field solver
+        current_fp_nodal, // Only used when using nodal current deposition
+        current_fp_vay,   // Only used when using Vay current deposition
+        current_buf, // Particles that are close to the edge of the MR patch (i.e. in the deposition buffer) deposit to this field.
+        current_store, // Only used when doing subcycling with mesh refinement, for book-keeping of currents
+        rho_buf, // Particles that are close to the edge of the MR patch (i.e. in the deposition buffer) deposit to this field.
+        rho_fp, // The charge density that is used as a source for the field solver (mostly for labframe electrostatic and PSATD)
+        F_fp, // Used for divE cleaning
+        G_fp, // Used for divB cleaning
+        phi_fp, // Obtained by the Poisson solver, for labframe electrostatic
+        vector_potential_fp, // Obtained by the magnetostatic solver
         vector_potential_fp_nodal,
         vector_potential_grad_buf_e_stag,
         vector_potential_grad_buf_b_stag,
@@ -44,19 +44,19 @@ namespace warpx::fields
         hybrid_current_fp_temp,
         hybrid_current_fp_ampere,
         hybrid_current_fp_external,
-        Efield_cp,
-        Bfield_cp,
-        current_cp,
-        rho_cp,
-        F_cp,
-        G_cp,
-        Efield_cax,
-        Bfield_cax,
-        E_external_particle_field,
-        B_external_particle_field,
-        distance_to_eb,
-        edge_lengths,
-        face_areas,
+        Efield_cp,  // Only used with MR. The field that is updated by the field solver at each timestep, on the coarse patch of each level
+        Bfield_cp,  // Only used with MR. The field that is updated by the field solver at each timestep, on the coarse patch of each level
+        current_cp, // Only used with MR. The current that is used as a source for the field solver, on the coarse patch of each level
+        rho_cp, // Only used with MR. The charge density that is used as a source for the field solver, on the coarse patch of each level
+        F_cp, // Only used with MR. Used for divE cleaning, on the coarse patch of each level
+        G_cp, // Only used with MR. Used for divB cleaning, on the coarse patch of each level
+        Efield_cax, // Only used with MR. Particles that are close to the edge of the MR patch (i.e. in the gather buffer) gather from this field
+        Bfield_cax, // Only used with MR. Particles that are close to the edge of the MR patch (i.e. in the gather buffer) gather from this field
+        E_external_particle_field, // Stores external particle fields provided by the user as  through an openPMD file
+        B_external_particle_field, // Stores external particle fields provided by the user as  through an openPMD file
+        distance_to_eb, // Only used with embedded boundaries (EB). Stores the distance to the nearest EB
+        edge_lengths, // Only used with embedded boundaries (EB). Indicates the length of the cell edge that is covered by the EB, in SI units
+        face_areas, // Only used with embedded boundaries (EB). Indicates the area of the cell face that is covered by the EB, in SI units
         area_mod,
         pml_E_fp,
         pml_B_fp,
@@ -73,7 +73,7 @@ namespace warpx::fields
         Bfield_avg_fp,
         Efield_avg_cp,
         Bfield_avg_cp,
-        Bold,
+        Bold, // Stores the value of B at the beginning of the timestep, for the implicit solver
         ECTRhofield,
         Venl
     );