[fvproblem][internaldirichlet] fix docu

What this MR does / why does DuMux need it:

Adjust the docs of the internal Dirichlet function fvproblem.hh regarding the required return type. An indexable container is required, but in e.g. porousmediumflow/compositional/primaryvariableswitch.hh, any is called on the result, which indicates that actually it requires an std::bitset.