Documentation fixes.

src/gravity/Default/gravity.h

@@ -164,9 +164,13 @@ __attribute__((always_inline)) INLINE static void gravity_init_gpart(
  * Multiplies the forces and accelerations by the appropiate constants.
  * Applies cosmological correction for periodic BCs.
  *
+ * No need to apply the potential normalisation correction for periodic
+ * BCs here since we do not compute the potential.
+ *
  * @param gp The particle to act upon
  * @param const_G Newton's constant in internal units.
  * @param potential_normalisation Term to be added to all the particles.
+ * @param Are we using periodic BCs?
  */
 __attribute__((always_inline)) INLINE static void gravity_end_force(
     struct gpart* gp, float const_G, const float potential_normalisation,