2cstokes2p2clocaloperator: removed upwind from the pm to the ff for the transport equation, since it does not work in case of two fluid phases. Added some more docu.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12372 2fb0f335-1f38-0410-981e-8018bf24f1b0