Feature/flux documentation effective stress

What this MR does / why does DuMux need it:

Description of the effective stresses for better readability of the code.

"Related to #1149 (closed)"

Notes for the reviewer A full description is also given in the poroelastic model. Should this be removed?

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)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• 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.