[multidomain] updated the docu for the dumux/multidomain files

dumux/freeflow/stokesnc/stokesncvolumevariables.hh

dumux/multidomain/2cstokes2p2c/2cstokes2p2clocaloperator.hh

.../multidomain/2cstokes2p2c/2cstokes2p2cnewtoncontroller.hh

dumux/multidomain/common/multidomainassembler.hh

dumux/multidomain/common/multidomainconvergencewriter.hh

dumux/multidomain/common/multidomainlocaloperator.hh

dumux/multidomain/common/multidomainmodel.hh

dumux/multidomain/common/multidomainnewtoncontroller.hh

dumux/multidomain/common/multidomainproblem.hh

dumux/multidomain/common/multidomainproperties.hh

dumux/multidomain/common/multidomainpropertydefaults.hh

dumux/multidomain/common/pdelablocaloperator.hh

dumux/multidomain/common/splitandmerge.hh

dumux/multidomain/common/subdomainproperties.hh

dumux/multidomain/common/subdomainpropertydefaults.hh

...omain/couplinglocalresiduals/2p2ccouplinglocalresidual.hh

...ain/couplinglocalresiduals/2p2cnicouplinglocalresidual.hh

...domain/couplinglocalresiduals/boxcouplinglocalresidual.hh

...n/couplinglocalresiduals/stokesnccouplinglocalresidual.hh

...couplinglocalresiduals/stokesncnicouplinglocalresidual.hh