[doxygen,freeflow]

- adaption to style of other models (replaced use of \nabla by use of div or grad)
- replaced use of mass fraction with mole fraction in diffusion term and added
  comment about use of binary diffusion
- fixed typos

[doxygen,material]
- fixed typos and line break



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15207 2fb0f335-1f38-0410-981e-8018bf24f1b0