[discretziation][doc] Use discMethod for all DiscretizationMethod template argument names

dumux/discretization/box/fourierslawnonequilibrium.hh

dumux/discretization/box/maxwellstefanslaw.hh

...x/discretization/cellcentered/mpfa/tensorlambdafactory.hh

dumux/discretization/cellcentered/tpfa/maxwellstefanslaw.hh

dumux/discretization/darcyslaw.hh

dumux/discretization/fickslaw.hh

dumux/discretization/fluxstencil.hh

dumux/discretization/fourierslaw.hh

dumux/discretization/fourierslawnonequilibrium.hh

dumux/discretization/maxwellstefanslaw.hh

dumux/discretization/staggered/freeflow/maxwellstefanslaw.hh

dumux/discretization/upwindscheme.hh

dumux/freeflow/navierstokes/fluxvariables.hh

dumux/freeflow/navierstokes/fluxvariablescache.hh

dumux/freeflow/navierstokes/localresidual.hh

dumux/freeflow/navierstokes/problem.hh

dumux/freeflow/navierstokes/staggered/fluxvariables.hh

dumux/freeflow/navierstokes/staggered/localresidual.hh

dumux/freeflow/navierstokes/vtkoutputfields.hh

dumux/freeflow/navierstokesnc/fluxvariables.hh