ENH: adding fields explanations for PDRFoam

applications/solvers/combustion/PDRFoam/PDRFoam.C

@@ -53,6 +53,18 @@ Description
     PDR (porosity/distributed resistance) modelling is included to handle
     regions containing blockages which cannot be resolved by the mesh.
 
+    The fields used by this solver are:
+
+    betav:  Volume porosity
+    Lobs:   Average diameter of obstacle in cell (m)
+    Aw:     Obstacle surface area per unit volume (1/m)
+    CR:     Drag tensor (1/m)
+    CT:     Turbulence generation parameter (1/m)
+    Nv:     Number of obstacles in cell per unit volume (m^-2)
+    nsv:    Tensor whose diagonal indicates the number to substract from
+            Nv to get the number of obstacles crossing the flow in each
+            direction.
+
 \*---------------------------------------------------------------------------*/
 
 #include "fvCFD.H"