fluid states: fix the documentation

IMHO it is perfectly okay to have some references to wikipedia for
basic stuff. Reason: Other online sources are also not reliable and
can usually found in the reference section of the wikipedia
article. (also, even for normal textbooks, there are the occational
errors.)

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7150 2fb0f335-1f38-0410-981e-8018bf24f1b0