minor: method doc to explain a lock of OdeLock

OpenSim/Region/Physics/OdePlugin/ODEPrim.cs

@@ -2378,6 +2378,8 @@ Console.WriteLine(" JointCreateFixed");
 
         public override void SetVolumeDetect(int param)
         {
+            // We have to lock the scene here so that an entire simulate loop either uses volume detect for all
+            // possible collisions with this prim or for none of them.
             lock (_parent_scene.OdeLock)
             {
                 m_isVolumeDetect = (param != 0);

OpenSim/Region/Physics/OdePlugin/OdeScene.cs

@@ -3668,8 +3668,8 @@ Console.WriteLine("AddPhysicsActorTaint to " + taintedprim.Name);
                 {
                     d.GeomSetCategoryBits(WaterGeom, (int)(CollisionCategories.Water));
                     d.GeomSetCollideBits(WaterGeom, (int)(CollisionCategories.Space));
-
                 }
+
                 geom_name_map[WaterGeom] = "Water";
 
                 d.Matrix3 R = new d.Matrix3();