Added doxygenable comment that Langevin friction coefficient is in units of...

Added doxygenable comment that Langevin friction coefficient is in units of collisions per picosecond (as opposed to picoseconds), which Sherm and I deduced by reading reference implementation and testing extreme values.

openmmapi/include/openmm/LangevinIntegrator.h

@@ -48,7 +48,7 @@ public:
      * Create a LangevinIntegrator.
      * 
      * @param temperature    the temperature of the heat bath (in Kelvin)
-     * @param frictionCoeff  the friction coefficient which couples the system to the heat bath
+     * @param frictionCoeff  the friction coefficient which couples the system to the heat bath, in units of collisions per picosecond
      * @param stepSize       the step size with which to integrator the system (in picoseconds)
      */
     LangevinIntegrator(double temperature, double frictionCoeff, double stepSize);