Incorrect commentary in dg-poisson demo

[Miro Kuchta]

Commentary in dg-poisson demo wrongly states that both Dirichlet and Neumann boundary conditions are imposed, while only the former one is actually set. Since setting bcs with DG is not trivial I'd also propose modifying the demo such that it shows how both Dirichlet and Neumann conditions can be set.

[Prof Garth Wells]

Fixed in 84851cf.