cleanup comments in fluidsystems

Some incorrect comments were corrected in dumux/material/fluidsystems. The merge request is made as draft, because it may lead to questions regarding the style of our comments/documentation. This is especially the case for commit: a85918d1 The main issue is which I want to mention here is: Class names, runtime parameters are sometimes highlighted (in generally, not only in this draft), but depending on the context, highlighting may be useful or not.