Feature/flux documentation shallow water

What this MR does / why does DuMux need it:

This merge request improves the documentation in shallowwaterflux.hh and shallowwaterviscousflux.hh header. Description about flux computation.

Notes for the reviewer

related to #1149 (closed)

Before you request a review from someone, make sure to revise the following points:

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.