[multidomain][dualnetwork][doc] Rename fancyFactor and improve documentation

What this MR does / why does DuMux need it:

See #1312 (closed)

Notes for the reviewer

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)