message: Add comment about central flux
diff: @@ -2742,7 +2742,8 @@ public:
  When combined with ConservativeConvectionIntegrator integrator, the
  coefficients alpha=1.0, beta=0.5 can be used to implement the upwind flux
  - and outflow boundary conditions in conservative form.
  + and outflow boundary conditions in conservative form. Setting beta=0.0
  + results in a centered flux.
  For the non-conservative formulation of convection using
  ConvectionIntegrator, the transpose of this form with alpha=-1.0, beta=0.5
  