STYLE: Added some comment to the parameters in 6dof bc

tutorials/multiphase/interDyMFoam/ras/floatingObject/0.org/pointDisplacement

@@ -37,7 +37,8 @@ boundaryField
         centreOfMass    (0.5 0.5 0.5);
         momentOfInertia (0.08622222 0.8622222 0.144);
         mass            9.6;
-        rhoInf          1;
+        rhoInf          1;                      // for forces calculation
+        // See sixDoFRigidBodyMotionState
         Q               (1 0 0 0 1 0 0 0 1);
         v               (0 0 0);
         a               (0 0 0);